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Compression and downsampling do different jobs
Downsampling reduces an image’s pixel dimensions by combining pixels. It can shrink a PDF when its images contain more resolution than the intended screen or print output needs, but it can also discard detail that would have been useful at a larger size. Adobe describes downsampling methods in its PDF settings overview.
Compression encodes image data to represent it in a PDF. A lossless method preserves the image data; a lossy method discards some information to reduce the encoded size. A PDF optimizer may offer separate controls for color, grayscale, and monochrome images, including whether to downsample, which images qualify, and which encoding to use. Changing one control does not necessarily make the other appropriate.
Choose lossless or lossy compression based on the images
Lossless: preserve image data
Choose a lossless method when retaining the original image data is the priority. Adobe says of Flate: “The Flate method is lossless, which means it does not remove data to reduce file size and so does not affect an image’s quality.” Flate is also known as ZIP compression. It may not reduce a particular PDF as much as a lossy method, and the amount of reduction depends on the source content.
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Lossy: smaller files at the cost of some information
JPEG-style compression can reduce file size by discarding image information, so aggressive settings can soften fine detail or introduce visible artifacts. Adobe’s PDF Optimizer guidance recommends JPEG or JPEG 2000 for images with gradual color transitions, such as photographs, and ZIP for illustrations with solid or flat-color areas. Treat this as a content-based starting point, not a universal ranking: inspect the result at the size and on the device that matter.
| Image or requirement | Practical starting point | Trade-off to check |
|---|---|---|
| Photographs or other gradual color transitions | JPEG or JPEG 2000, following Adobe’s Optimizer guidance | Lossy settings can degrade detail or create artifacts; compare the result with the original. |
| Illustrations with flat or solid colors | ZIP/Flate, following Adobe’s Optimizer guidance | Lossless encoding preserves image data, but the file-size change depends on the source. |
| Image data must be retained | A lossless option such as ZIP/Flate | Do not assume it will achieve the smallest file. |
| Scanned PDF where efficient color compression is the priority | Adobe’s scanned-PDF guidance recommends JPEG 2000 | Adobe says JPEG 2000 is not recommended for PDF/A files; confirm the required conformance and workflow. |
Set image resolution for the destination
Resolution affects how much image detail is available at output size. Adobe’s September 23, 2025 PDF settings overview gives these examples of image resolution for particular output devices:
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| Adobe’s output-device example | Example image resolution |
|---|---|
| 300 dpi laser printer | 120 ppi |
| 600 dpi laser printer | 170 ppi |
| 1200 dpi imagesetter | 240 ppi |
| 2400 dpi imagesetter | 300 ppi |
These are Adobe’s examples, not universal minimums or print requirements. Ask the print provider for its specifications when preparing a production job. For screen viewing, printing, or a specific workflow, choose a target appropriate to that destination rather than automatically retaining an unnecessarily high source resolution or applying a generic preset.
The same Adobe settings overview says its documented resolution control ranges from 72 to 4000. That is the range of the control described there—not a recommended range for every PDF. Adobe notes that higher resolution may increase file size and processing time.
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Special care with scanned and archival PDFs
Scanned PDFs often expose resolution or DPI and image-optimization controls. Adobe’s guidance for improving scanned PDFs recommends JPEG 2000 for efficient color image compression and says ZIP is an option when lossless compression is preferred. It also warns that JPEG 2000 is not recommended for PDF/A files.
If a PDF must meet PDF/A or another archival, regulatory, or delivery requirement, preserve the required conformance and validate the output in the intended workflow. Do not optimize only for byte count. The cited Adobe guidance does not establish compatibility details for every PDF/A part or every vendor implementation.
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A practical way to choose and verify settings
- Identify the destination. Decide whether the PDF is for screen reading, ordinary printing, or a print-production workflow with provider-specific requirements.
- Check the source images. Determine whether they are photographs, flat-color illustrations, scans, or a mixture. Consider downsampling only if the existing image resolution exceeds what the destination needs.
- Choose the fidelity trade-off. Use lossless ZIP/Flate when preserving image data matters most. Consider JPEG or JPEG 2000 for gradual-tone images when lossy compression is acceptable; use ZIP as a practical starting point for flat-color artwork.
- Respect format requirements. Confirm the required PDF version or PDF/A conformance and that the intended reader or workflow supports the selected method.
- Optimize a copy, then inspect it. Compare file size and quality rather than assuming a particular reduction. Check small text, edges, gradients, and photographs at the intended viewing or printing scale. Keep the original if the result loses needed detail or fails the workflow.
Adobe’s Optimizer can be configured to optimize images only when doing so reduces file size, according to its Optimizer settings guidance. That is an option in Adobe Acrobat, not a universal control in every PDF application; labels and available settings vary by software.
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