1. The download was interrupted
This is the most common cause. If the connection dropped while you were downloading an attachment, the file is incomplete: the end of the PDF, which contains its index, is missing. Check the file size: a file of 0 KB or much smaller than expected is incomplete. Download it again, ideally on a stable Wi-Fi connection.
2. The file is damaged
A USB key removed too early, a failing memory card or a transfer error can corrupt a PDF. The structure of the file is broken, but the content is often still there. Try Repair PDF: it rebuilds the internal structure and recovers every readable page.
3. It is protected by a password
A PDF encrypted with a password cannot be opened without it — some readers simply show an error instead of asking for it. If you know the password, you can remove it once and for all with Unlock PDF. If you do not know it, ask the sender: no honest tool can open it without the password.
4. It is not really a PDF
Sometimes a Word document, an image or a web page is saved with a .pdf extension by mistake. Open the file with another program (Word, an image viewer or a text editor): if the first characters are not %PDF, it is not a PDF. Ask the sender to export it again properly.
5. Your PDF reader is outdated or faulty
Try another reader before concluding that the file is broken. Every modern browser (Chrome, Edge, Firefox, Safari) opens PDFs: drag the file into a browser window. If it opens there, update or reinstall your usual reader.
6. The file is too large for your phone
A 200-page scan of 150 MB can crash a phone’s viewer. Open it on a computer and reduce its size with Compress PDF, or split it into smaller parts with Split PDF.
7. The email or cloud service blocked it
Some company email filters remove or rename attachments they consider risky. If the file opens for the sender but not for you, ask them to share it through a cloud link or to compress it into a ZIP archive.
How to avoid the problem next time
- Keep a copy of important documents in two places (computer and cloud).
- Always eject USB keys properly before removing them.
- For long-term archiving, convert important files to PDF/A, the ISO format designed to remain readable for decades.