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To merge complete PDF files in Java, use Apache PDFBox’s PDFMergerUtility. To include only selected pages, create a destination PDDocument and call importPage for each page you want. For PDFs supplied as InputStreams, the right loading code depends on whether your project uses PDFBox 2.x or 3.x: PDFBox 3 uses Loader and RandomAccessReadBuffer.
The examples below are explicitly for PDFBox 3 unless a section says otherwise. They cover whole-document concatenation, page selection, and stream-backed input, plus the version and resource-handling decisions that commonly cause problems.
How do I merge complete PDF files in Java?
For straightforward concatenation, create a PDFMergerUtility, add each source file in the order you want it to appear, set a destination, and call mergeDocuments. This PDFBox 3 example keeps the inputs and output separate:
import java.nio.file.Path;
import org.apache.pdfbox.multipdf.PDFMergerUtility;
PDFMergerUtility merger = new PDFMergerUtility();
merger.addSource(Path.of("part-1.pdf").toFile());
merger.addSource(Path.of("part-2.pdf").toFile());
merger.setDestinationFileName("combined.pdf");
merger.mergeDocuments(null);addSource calls establish the document order: in this example, the pages from part-1.pdf come before the pages from part-2.pdf. Add further sources in their intended order. The destination should be a different file from every input; writing over an input can corrupt it.
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The utility is intended to merge a list of PDF documents into a new document. It supports file sources as well as RandomAccessRead sources, and can write to a destination filename or an OutputStream. If your application needs to control output handling, set an output stream rather than a destination filename, and manage that stream’s lifecycle in your code.
What does mergeDocuments(null) mean?
In the PDFBox 3 API, mergeDocuments accepts a stream-cache factory. The example passes null, as in the documented practical pattern, rather than selecting an explicit cache strategy. For large or memory-sensitive jobs, choose and pass a cache factory deliberately instead of assuming that unlimited in-memory processing is appropriate. The available evidence does not establish a maximum safe file size or a memory threshold, so do not treat any particular file size as a universal cutoff.
When should I use the merger utility?
Use it when the requirement is to concatenate entire PDFs in a defined order. It keeps the application code concise and avoids manually creating a destination and importing every page. If you need to omit, reorder, or repeat individual pages, use the page-level approach instead.
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Load a source document, create a new destination document, and import only the pages you want. The page indexes in the loop are zero-based: page index 0 is the first page, 2 is the third, and 5 is the sixth.
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import org.apache.pdfbox.Loader;
import org.apache.pdfbox.io.RandomAccessReadBufferedFile;
import org.apache.pdfbox.pdmodel.PDDocument;
try (PDDocument source = Loader.loadPDF(
new RandomAccessReadBufferedFile("input.pdf"));
PDDocument destination = new PDDocument()) {
int[] wanted = {0, 2, 5};
for (int index : wanted) {
destination.importPage(source.getPage(index));
}
destination.save("selected-pages.pdf");
}This creates a new PDF containing the selected pages in the order listed in wanted. Change the array to select different pages; for example, {5, 2, 0} imports those pages in reverse order. To import all pages from a source, iterate through source.getPages() and import each page.
What does importPage copy?
importPage copies page contents into the destination and creates a new page object. Its deep copy means the imported page’s resources remain available when the source document is closed. This makes it suitable for building a separate output document from chosen pages rather than leaving the output dependent on the source remaining open.
What can happen to annotations and page references?
Annotations may link to pages outside the set you import. Those references can make the destination unexpectedly large, and an annotation’s target may not be part of the selected-page output. Review such references when working with annotated or linked PDFs; remove irrelevant references if the output should not retain them. Do not assume that choosing a small number of visible pages guarantees a proportionally small output file.
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In PDFBox 3, wrap an InputStream in RandomAccessReadBuffer and pass it to Loader.loadPDF. You can then import pages into a destination document as usual. This example reads a source from a file stream and writes the selected complete document to another path:
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import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import org.apache.pdfbox.Loader;
import org.apache.pdfbox.io.RandomAccessReadBuffer;
import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.PDPage;
Path inputPath = Path.of("input.pdf");
Path outputPath = Path.of("output.pdf");
try (InputStream in = Files.newInputStream(inputPath);
PDDocument source = Loader.loadPDF(new RandomAccessReadBuffer(in));
PDDocument destination = new PDDocument()) {
for (PDPage page : source.getPages()) {
destination.importPage(page);
}
destination.save(outputPath.toFile());
}RandomAccessReadBuffer stores the stream’s data in memory. That is convenient when an input arrives as a stream rather than a file path, but it is a real resource trade-off: the input bytes are held in memory. For larger jobs, consider how your application manages buffering and caching, and avoid assuming the stream-backed path has negligible memory cost.
How can I concatenate multiple streams?
For a page-level workflow, load each stream as a PDFBox 3 source, import the required pages into one destination document, then save the destination once. The source documents and streams should be managed with try-with-resources, and each source must remain available while its pages are being imported.
For whole-document concatenation, PDFBox 2.x offers PDFMergerUtility.addSource(InputStream) and addSources(List<InputStream>). Do not copy those overloads into a PDFBox 3 project: PDFBox 3 uses RandomAccessRead sources and the Loader loading API. If a stream must be reused or retained elsewhere, check the ownership and closing behavior of the APIs used in your version and ensure the application closes all resources it owns.
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What is different between PDFBox 2.x and 3.x?
The key migration difference is how documents are loaded and how stream-backed sources are represented. Code that compiles against one major version may not compile against the other, so verify the version declared by your project before choosing an example.
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| Task | PDFBox 2.x | PDFBox 3.x |
|---|---|---|
| Load a PDF | The older PDDocument.load(...) family is used. |
Use Loader.loadPDF(...); PDFBox 3 removed the old PDDocument.load(...) family. |
| Whole-document merge from files | PDFMergerUtility is available; use signatures that match the 2.x dependency in the project. |
PDFMergerUtility accepts file sources; the example above uses addSource(File). |
| Whole-document merge from streams | addSource(InputStream) and addSources(List<InputStream>) are available. |
Adapt stream-backed input to RandomAccessRead; do not use the 2.x stream overload as though it were the 3.x API. |
| Stream-backed document loading | Use the 2.x loading APIs and types applicable to that version. | Wrap the stream in RandomAccessReadBuffer and load it with Loader. |
| Java requirement | Not stated here. | PDFBox 3 requires Java 8 or later. |
The table intentionally does not give a complete PDFBox 2.x code sample: loading and merge method signatures depend on the 2.x API, and mixing them with PDFBox 3 examples is a common source of compile errors. Keep each project’s imports, method calls, and dependency on the same major version. For migration work, consult the official PDFBox 3 migration guide and the API documentation for the exact version in use.
How should I choose between whole-document merge and page import?
The choice depends on whether you need to preserve a complete set of source documents or construct a new document from selected pages. These approaches solve different jobs rather than offering interchangeable syntax.
| Need | Best fit | Important trade-off |
|---|---|---|
| Append every page of each input in order | PDFMergerUtility |
Less page-by-page code; choose the merge mode and caching behavior deliberately. |
| Keep only chosen pages | Destination PDDocument plus importPage |
Requires explicit indexes and ordering; page-linked annotations may refer to omitted pages. |
Work with an InputStream in PDFBox 3 |
RandomAccessReadBuffer plus Loader, then import or merge as appropriate |
The buffer stores input data in memory. |
| Use legacy PDF structure behavior | PDFBox merger in legacy mode when required for the structure-tree case | Source documents remain open during the merge. |
| Release source documents earlier where applicable | PDFBox merger optimized-resources mode | Optimized mode does not merge every document element. |
PDFBox’s merger exposes PDFBOX_LEGACY_MODE and OPTIMIZE_RESOURCES_MODE. Optimized mode can release source documents earlier, but it does not merge every document element; legacy mode keeps files open and is required for some structure-tree cases. Do not choose a mode solely because its name sounds faster or more complete. Check whether your PDFs contain structures your application must retain, and verify output behavior with representative documents.
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How do I avoid resource, concurrency, and output-file problems?
PDF merging involves file handles, document objects, and potentially substantial buffered data. Handle these explicitly to prevent leaks, corruption, and unsafe concurrent access.
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- Close every document. Put each
PDDocumentin try-with-resources. The PDFBox FAQ requires closing allPDDocumentobjects. - Do not write over an input. Use a destination path that differs from every source path; saving over an input may corrupt it.
- Do not access one document from multiple threads at once. The PDFBox FAQ states, “Only one thread may access a single document at a time.” Parallelize independent jobs only when each job has its own document objects.
- Choose caching for the workload. Pass an explicit stream-cache factory for large jobs when appropriate; no benchmark or universal safe memory limit is established here.
- Account for stream buffering. PDFBox 3’s
RandomAccessReadBufferholds the InputStream data in memory. - Validate page indexes. An index outside the source document’s page range cannot identify a page to import. Check the source page count before processing untrusted or variable input.
- Consider document features, not only visible pages. Annotations with references to pages outside the selected set can affect the output.
How can I check the expected merge behavior?
PDFBox 3 also provides a command-line merge equivalent, useful for checking that the intended result is simple concatenation before integrating the same operation into application code. The documented command pattern is java -jar pdfbox-app-3.y.z.jar merge -o=outfile -i=infile -i=infile. Replace the illustrative jar name and file names with the actual PDFBox app jar and paths for your installation; the command does not itself prove that a complex document’s forms, annotations, or structure are preserved as your application requires.
If the command-line output differs from your Java output, first confirm that both runs use the same PDFBox major version, input order, and source files. For page selection, compare the exact zero-based indexes and their order. For errors involving streams, confirm that the code uses the API family for the project’s PDFBox version.
Which approach should I use?
Use PDFMergerUtility for uncomplicated concatenation of complete PDFs. Use a destination PDDocument with importPage when the result must include only particular pages or must reorder them. In PDFBox 3, load stream-backed inputs through RandomAccessReadBuffer and Loader; keep every document closed safely, write to a separate output file, and select merge behavior with awareness of document structures and memory use.
Frequently Asked Questions
Can I merge encrypted PDFs with PDFBox?
The examples here do not establish password handling or the conditions for merging encrypted inputs. Check the PDFBox API documentation for the exact version and the permissions and credentials associated with each source before designing that workflow.
Does merging PDFs preserve bookmarks, forms, and document metadata?
Do not assume every document-level feature is preserved identically by every merge mode. In particular, PDFBox documents limits for optimized-resources mode and special structure-tree cases for legacy mode; test the features your output must retain.
Can I merge PDFs without creating a temporary output file?
The merger utility can target an OutputStream, so a filesystem destination is not the only output option. Your application remains responsible for managing that stream and for deciding where the resulting bytes should go.
Is PDFBox thread-safe for parallel PDF merging?
The PDFBox FAQ’s restriction is per document: only one thread may access a single document at a time. This does not by itself establish that every shared object or surrounding application component is safe to use concurrently.
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