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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchA 1 KB file is not automatically a damaged PDF. Your script may have saved an HTML error or login page, a redirect response, or only part of the file. Inspect the HTTP status, final URL, headers, and first bytes before changing libraries or adding request headers.
Diagnose the response before diagnosing the PDF
The filename and .pdf extension describe what you called the output, not what the server returned. A small response can be an access-denied page, authentication form, redirect endpoint, incomplete transfer, or an actual (but unusually small) document.
- Check the status code and final URL.
- Inspect
Content-TypeandContent-Length. - Examine the first bytes or a short text preview.
- Compare the bytes written with a trustworthy declared length, when one exists.
Requests documents checking status_code or calling raise_for_status(); receiving a response object alone does not prove that the request succeeded.
A safe Requests download pattern
Stream the body, open the destination in binary mode, and close the response reliably:
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import requests
url = "https://example.com/file.pdf"
with requests.get(url, stream=True, timeout=30) as response:
response.raise_for_status()
print("Final URL:", response.url)
print("Status:", response.status_code)
print("Content-Type:", response.headers.get("Content-Type"))
print("Content-Length:", response.headers.get("Content-Length"))
with open("download.pdf", "wb") as output:
for chunk in response.iter_content(chunk_size=64 * 1024):
if chunk:
output.write(chunk)Requests recommends iter_content() for streamed downloads and writing chunks to a file opened with 'wb'. It handles gzip and deflate transfer encodings. By contrast, Response.raw exposes the raw stream and is not the usual file-writing interface.
With stream=True, Requests obtains the headers first and leaves the connection open until you consume the body or close the response. The with statement ensures closure even when an error occurs.
Inspect what the short file contains
Run a separate diagnostic request, or inspect the saved bytes:
from pathlib import Path
path = Path("download.pdf")
data = path.read_bytes()
print("Bytes written:", len(data))
print("First 16 bytes:", data[:16])
print("Text preview:", data[:300].decode("utf-8", errors="replace"))A normal PDF begins with the ASCII signature %PDF-. If the preview starts with <!DOCTYPE html>, <html>, or a message such as “access denied,” you saved a web page rather than a PDF. A URL that ends at a sign-in page can look successful at the HTTP level while still returning HTML. If the first bytes do start with %PDF-, compare the written length with the server’s declared length and investigate truncation.
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Do not treat a missing or incorrect Content-Length as proof either way. Chunked responses may omit it, and a server can send an inaccurate value.
Interpret the common failure branches
HTML error or access page
A 4xx or 5xx status should be handled by raise_for_status(). Some services nevertheless return a 200 status for an application-level error page. Confirm the final URL, then use the legitimate document URL and the required authentication or session. A browser-style User-Agent is not a general fix for permissions, expiring links, cookies, or signed URLs.
Redirect or login endpoint
Print response.url after redirects. If it points to a different host, a sign-in route, or a download handler rather than the document, obtain an authorized URL or preserve the required session. Do not assume that the original URL’s extension reflects the final response.
Partial transfer
If the bytes begin with %PDF- but are fewer than a reliable Content-Length, the connection may have been interrupted or the server may have supplied inconsistent metadata. Record both values, retry with a stable connection, and investigate the server or proxy. The fact that a file was created does not establish that the body was complete.
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application/pdf is the expected media type, but headers are hints rather than proof. Conversely, some servers mislabel valid files. Use the status, final URL, header values, and magic bytes together.
Using Python’s standard library instead
urllib.request.urlretrieve() provides a direct URL-to-file helper:
from urllib.request import urlretrieve
urlretrieve("https://example.com/file.pdf", "download.pdf")Python 3.13 documentation says urlretrieve() raises ContentTooShortError when it detects fewer bytes than a supplied Content-Length, such as after an interrupted download. If the server sends no Content-Length, it cannot perform that size check and simply returns the file. The exception still does not prove that a file is a valid PDF, nor does it prove a server’s length header is correct.
Requests or urlretrieve()?
| Need | Requests | urllib.request.urlretrieve() |
|---|---|---|
| Stream and process chunks | Use iter_content() with explicit chunk size and binary output. |
Direct file-copy helper; less control over the body-processing loop. |
| Inspect response details | Exposes status, final URL, headers, and streamed content directly. | Convenient for copying, but response inspection is less central to the helper. |
| Short-read check | Compare written bytes with a usable expected length yourself. | Raises ContentTooShortError when a supplied length reveals a short read; no length means no check. |
| Connection cleanup | Consume the body or close the response; a context manager handles this. | Managed by the helper. |
A practical checklist
- Print the HTTP status and final URL.
- Call
raise_for_status()before writing the file. - Print
Content-TypeandContent-Length. - Save with binary mode and streamed chunks.
- Check that the first bytes are
%PDF-. - Compare actual and declared lengths only when the declared length is usable.
- If the body is HTML, fix the URL, authentication, session, or access requirement rather than repeatedly changing download code.
- If the body is a truncated PDF, retry and examine transfer interruptions or server-side length errors.
Why the 1 KB figure cannot identify the cause
The title supplies no URL, status code, headers, or bytes, so no single explanation is established. A historical Requests issue opened on 27 June 2019 described 2,583 returned bytes against a declared Content-Length of 66,892,906—an example of a possible partial transfer or server metadata problem, not a frequency statistic or a diagnosis of every 1 KB download.
Inspecting the actual response is therefore the decisive next step: it tells you whether to correct access, follow the proper endpoint, or recover an incomplete transfer.
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