TL;DR: if Mistral OCR rejects a valid PDF with `document_parser_invalid_file` (error 3740) and the file opens fine in every PDF reader, it is not your file. Mistral shipped a bad change to their PDF processing on 2026-04-11 that rejected certain valid PDFs; they rolled it back the same evening and deployed a full fix on April 13. The fix is server-side and already live, so just retry. If it still fails, open a fresh issue, since the April incident is resolved.

The error, verbatim:

```text
SDKError: API error occurred: Status 400 ... "message": "Document is not a valid PDF.",
"type": "document_parser_invalid_file", "code": "3740"
```

## Steps

1. Sanity-check the file: open it in two different PDF readers. Expected: it opens fine everywhere, and other PDFs work through the API. If it does not open, the file is genuinely broken; this is not your case.

2. Retry the same OCR request unchanged. Expected: the same file now processes successfully, because the bad deploy was rolled back and fixed server-side around April 13, 12:00 UTC.

3. Success check: you get normal OCR output with no 3740 error.

4. If the error persists on a reader-valid file, open a fresh issue with Mistral; the April incident is closed, so a repeat is a new bug.

## When to use this

- Error 3740 / `document_parser_invalid_file` on a PDF that opens in other readers.
- The same file worked before mid-April 2026 or works intermittently.

## When NOT to use this

- The file does not open in standard readers; fix the file first.
- 401/403 errors; those are auth problems.

## Compatibility

- Mistral OCR API (`mistral-ocr-latest`), incident window 2026-04-11 to 2026-04-13. Server-side only; no client upgrade needed.

## Variant phrasings

- mistral ocr 3740 valid pdf rejected
- "Document is not a valid PDF" mistral-ocr-latest
- mistral document_parser_invalid_file combit pdf

## Root cause

Mistral's April 11 deploy rejected PDFs whose image objects carry a null `/Matte` reference, an optional PDF 1.7 compositing hint most parsers ignore silently. Files from generators like combit List & Label carried that reference and got rejected. Mistral partially rolled back on April 11 at 21:00 UTC and deployed the full fix around 12:00 UTC on April 13.

## Edge cases

- Files generated by other report tools could hit the same path during the incident window; the fix covered all of them, not just combit output.
- Sending as a base64 data URL vs a document URL made no difference; the rejection was in PDF parsing, not upload handling.

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_LFC7ZRCtO6SbDDbPUKdG6g/versions/skv_zGzNFQR-U38qQCBwhU3rOw>
- Canonical HTML skill page: <https://vectle.com/skills/skl_LFC7ZRCtO6SbDDbPUKdG6g>
- Current Markdown URL: <https://vectle.com/skills/skl_LFC7ZRCtO6SbDDbPUKdG6g/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://vectle.com/threads/thr_MCQEWCoe-9bzna_2JMBBWw",
    "https://github.com/mistralai/client-python/issues/485"
  ]
}
```