If a JavaScript API call returns an empty PDF, first check whether you received PDF bytes at all. Read a successful response with response.blob() or response.arrayBuffer()—not response.text() or response.json()—and verify the status, Content-Type, and byte count before saving it. A 200 response can still contain an HTML error page or JSON, and a CORS-opaque response can produce a zero-byte Blob.
Start by checking what the API actually returned
Open your browser’s DevTools, select the Network tab, and inspect the request that fetches the PDF. Check the final response after redirects, not just the initial request. Record the HTTP status, response type, Content-Type, Content-Length if available, and the downloaded size. Also check whether a preflight request occurred and whether it succeeded.
A 200 status only means the server reported success at the HTTP level. It does not prove the response body is a PDF. If the Content-Type is application/json or text/html, the server may have returned an error object, login page, proxy error, or other content. Don’t save that response as a PDF. Fetch’s response methods and status handling are documented by MDN.
When the body is readable, inspect its first bytes as a diagnostic: a PDF normally begins with the ASCII signature %PDF. If you see {, <, or an error message instead, the response is not the expected PDF. This check can help locate a wrong response, but it is not a complete test of PDF validity.
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Download a PDF with Fetch
For a browser download, use blob() and check the response before creating a download link. If the server returns a useful error body on failure, read that body as text before throwing.
const response = await fetch('/api/report');
if (!response.ok) {
const message = await response.text();
throw new Error(`HTTP ${response.status}: ${message}`);
}
const type = response.headers.get('content-type') || '';
if (!type.includes('application/pdf')) {
throw new Error(`Expected PDF, received ${type || 'no Content-Type'}`);
}
const blob = await response.blob();
if (blob.size === 0) {
throw new Error('PDF body is empty');
}
const url = URL.createObjectURL(blob);
const link = document.createElement('a');
link.href = url;
link.download = 'report.pdf';
document.body.appendChild(link);
link.click();
link.remove();
URL.revokeObjectURL(url);The browser’s Response provides body readers for different formats, including blob() and arrayBuffer(); choose the one that fits what you need to do with the bytes. See MDN’s Response reference. A Blob is convenient for downloads and object URLs. Use an ArrayBuffer when you need to inspect bytes, pass them to a PDF parser, or send them to another binary sink.
For byte inspection, read the response once as an ArrayBuffer and examine a short prefix. Do not first call blob() and then try to read the same response again: the body is consumed by a reader.
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const response = await fetch('/api/report');
if (!response.ok) throw new Error(`HTTP ${response.status}`);
const bytes = await response.arrayBuffer();
const prefix = new TextDecoder().decode(bytes.slice(0, 8));
console.log({ byteLength: bytes.byteLength, prefix });Set Axios to receive binary data
Axios needs an explicit binary response type. In a browser, use blob; when you need raw bytes in Node.js or for byte-level processing, use arraybuffer. Without the right response type, the returned data may not be handled as the binary body you expect. An Axios issue documents the symptom of response.data appearing as an empty object; treat it as an example of a failure report, not as a current specification for Axios behavior: Axios issue #1392.
const { data, headers, status } = await axios.get('/api/report', {
responseType: 'blob',
headers: { Accept: 'application/pdf' }
});
if (status < 200 || status >= 300) {
throw new Error(`HTTP ${status}`);
}
if (!data || data.size === 0) {
throw new Error('Empty PDF body');
}
const type = data.type || headers['content-type'] || '';
if (type && !type.includes('application/pdf')) {
throw new Error(`Expected PDF, received ${type}`);
}
const blob = data.type ? data : new Blob([data], { type: 'application/pdf' });
const url = URL.createObjectURL(blob);
const link = document.createElement('a');
link.href = url;
link.download = 'report.pdf';
link.click();
URL.revokeObjectURL(url);For Node.js or other byte-oriented work, use responseType: 'arraybuffer', then pass the bytes to the destination without converting them to text. An HTTP status and a nonzero size are useful checks, but still confirm the response type and, when debugging, inspect a byte prefix.
Rule out an opaque CORS response
A cross-origin request can fail in a way that looks like an empty PDF if the browser gives JavaScript an opaque response. MDN states that an opaque response has status 0, inaccessible headers, and a null body; converting it to a Blob produces size 0 and an empty type. That Blob is unusable for URL.createObjectURL(). See MDN’s Blob method reference.
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Check the Network tab and console for CORS errors, then verify that the API permits the requesting origin. If the request uses credentials, its mode and credentials must also agree with the server’s CORS policy. A cross-origin response may also hide headers from JavaScript unless the server exposes them; compare what DevTools shows with what response.headers.get() returns.
To separate a CORS problem from a PDF-generation problem, request the endpoint directly where appropriate or make the request through a same-origin backend proxy. Do not use mode: 'no-cors' as a fix: it produces an opaque response that JavaScript cannot read as a usable PDF.
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Forward the bytes correctly from your server
If a backend or serverless route sits between the PDF provider and the browser, it must forward the actual bytes unchanged. Read the upstream body as an ArrayBuffer, convert it to a Node.js Buffer, and send it as bytes. Do not pass PDF data through res.json() or JSON.stringify().
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const upstream = await fetch(PDF_URL, options);
if (!upstream.ok) {
const error = await upstream.json();
return res.status(upstream.status).json(error);
}
const bytes = await upstream.arrayBuffer();
res.setHeader('Content-Type', 'application/pdf');
res.setHeader('Content-Disposition', 'attachment; filename=document.pdf');
res.send(Buffer.from(bytes));Use Content-Type: application/pdf to identify the body as a PDF. Content-Disposition: attachment; filename=... suggests download behavior and a filename; it does not turn a non-PDF body into a PDF. Preserve the upstream error status and body while diagnosing failures so the client can distinguish a generation error from a download-handling error.
Official examples from RenderPDF show checking upstream success, reading bytes with arrayBuffer(), and returning the PDF with these headers. They also warn: “Never expose API keys in client-side code. Use a backend proxy to make API calls.”
Choose between a direct browser call and a backend proxy
| Approach | What to consider |
|---|---|
| Direct browser request | Requires a server CORS policy that permits the browser origin. Any API key embedded in client-side code is exposed to users. |
| Backend proxy | Keeps API credentials on the server and gives you a place to log status, validate headers and bytes, and control how errors reach the client. The proxy must preserve the PDF bytes and set appropriate response headers. |
Fetch and Axios can both work for binary responses. The practical differences are the body reader or Axios responseType, whether code runs in a browser or Node.js, what headers CORS exposes, how error bodies are handled, and who revokes a created object URL. Pick based on the runtime and the surrounding API flow, not on the assumption that one library makes invalid server responses valid.
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Common empty-PDF symptoms and fixes
- Status 200, but the file is blank or unreadable: Check Content-Type and the first bytes. The endpoint may have returned HTML or JSON instead of a PDF.
- Blob size is zero and type is empty: Check for an opaque CORS response, a zero-byte server response, or a failed upstream request. For an opaque response, fix the server’s CORS policy or route through a same-origin backend.
- Axios shows an empty object: Set
responseType: 'blob'in a browser orresponseType: 'arraybuffer'when handling raw bytes. - The API returns JSON on errors: Check
response.okbefore reading a successful response as a PDF. Read or preserve the error body separately rather than naming it with a.pdfextension. - The browser reports a CORS error or status 0: Confirm allowed origins, credentials, and preflight behavior. A browser CORS failure cannot be repaired by trying to parse its opaque body.
- The proxy returns a damaged file: Ensure it forwards the ArrayBuffer or Buffer directly, sets
application/pdf, and does not stringify or JSON-encode the bytes. - Content-Type is unavailable in JavaScript: Check whether the response is cross-origin and whether the server exposes the needed headers. DevTools may show details that the page’s JavaScript cannot access.
Or skip the browser setup
If your goal is to capture a webpage as a PDF, ScreenshotNeo can return a PDF from one API request. The JavaScript examples above address fetching and forwarding an existing PDF; ScreenshotNeo is an alternative when the PDF should be rendered from a URL.
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);See the ScreenshotNeo API documentation for request options. Cookie banners, popups, and chat widgets are removed before the shot; bot checks, blank pages, and failed loads are never billed. Its MCP server lets AI agents take screenshots. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000. Sign up for free.
FAQ
Should I use blob() or arrayBuffer()?
Use blob() for a browser download or object URL. Use arrayBuffer() when you need to inspect or pass around raw bytes, including in a server-side proxy.
Does a 200 response mean the PDF is valid?
No. It means the HTTP request succeeded according to the server’s status code. Verify the response body type and size, and inspect bytes when troubleshooting.
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