When Gmail does not show a preview for an attachment, the file is often fine. Work through these in order.
1. Reload and reopen
Press Ctrl+R (Cmd+R on a Mac), open the email again and click the attachment card (not the download arrow).
2. Rule out your extensions
Open Gmail in an Incognito window (Ctrl+Shift+N). Extensions are off there by default. If the preview works in Incognito, turn your extensions off in chrome://extensions and back on one at a time to find the one causing it. Ad blockers and PDF extensions are the usual suspects. For Acrobat, use the Gmail checkbox steps in Adobe Acrobat extension and Gmail PDFs.
3. Check Chrome's PDF setting
Go to chrome://settings/content/pdfDocuments and choose Open PDFs in Chrome.
4. Check the file
- Files over 25 MB arrive as Google Drive links, not attachments. Open the Drive link instead.
- If the same file fails on every device, ask the sender to send it again.
5. Password-protected PDFs
Gmail's native viewer prompts for a password on protected PDFs. In version 1.1 and later, Attachment Preview does too, inside the overlay so you stay in the thread: you see an inline Password form with an Open button. A wrong password shows Incorrect password, try again. The password is used only to open that file and is never stored, logged or transmitted.
6. Preview PDFs and images with Attachment Preview
Attachment Preview is a free Chrome extension that adds a Preview button under PDF and image attachments in Gmail. It opens the file in its own built-in viewer (Mozilla's pdf.js for PDFs) on top of your inbox: you stay in the thread, get a page counter, and press Esc to close. Nothing downloads. It also works in compose, where Gmail otherwise downloads when you click an attachment chip.
Questions
Is my attachment damaged if Gmail can't preview it?
Usually not. Preview problems are often caused by the browser, an extension, or a temporary Gmail issue. If it opens after a reload, in Incognito, or after downloading, the file is fine.
Can Attachment Preview open a password-protected PDF?
Yes, in version 1.1 and later. Enter the password in the overlay (Gmail's own viewer also prompts). The password is used only to open that file and is never stored, logged or transmitted.
Is it safe to download the file instead?
Only if you trust the sender. Gmail scans attachments, but be careful with files you were not expecting.
Related: Gmail PDF preview not working · Gmail "No preview available"