You click a PDF in Gmail and nothing useful happens: the preview spins, Chrome opens a new tab, Acrobat takes over, or the file downloads. Work through these in order. Most people are fixed by step 2 or 3.

1. Reload Gmail and try again

Press Ctrl+R (Cmd+R on a Mac). Gmail updates itself in the background, and a stale page is the most common cause of a preview that never loads.

2. Test in an Incognito window

Open an Incognito window (Ctrl+Shift+N), sign in to Gmail and open the same email. Extensions are off in Incognito by default.

It works in Incognito: one of your extensions is the cause. Go to chrome://extensions, turn them all off, then turn them back on one at a time until the preview breaks again. Ad blockers and PDF tools are the usual suspects.
It still fails: skip to step 4.

3. Stop Acrobat from taking over Gmail PDFs

With Adobe Acrobat for Chrome installed, Gmail's native PDF click is taken over and opens a chrome-extension:// tab. To keep Acrobat but restore Gmail's preview:

Click the puzzle icon in Chrome's toolbar.
Click Adobe Acrobat, then the three-dot menu, then Options.
On Adobe Acrobat preferences > General, under Open PDFs in Acrobat from your browser, uncheck Gmail.
Reload Gmail.

Details: Adobe Acrobat extension and Gmail PDFs.

4. Check Chrome's PDF setting

Go to chrome://settings/content/pdfDocuments. If Download PDFs is selected, Chrome saves PDFs instead of showing them. Choose Open PDFs in Chrome.

5. Check the file itself

Very large files: Gmail sends files over 25 MB as Google Drive links, not attachments. Click the Drive link instead.
Spam folder: Gmail limits what you can do with messages in Spam. If the email is safe, click Report not spam and try again.
If the same file fails everywhere, ask the sender to export it again.

6. 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 its overlay: 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.

7. Preview inside Gmail with Attachment Preview

Attachment Preview is a free Chrome extension that adds a Preview button under every PDF in Gmail. It opens the PDF in its own built-in viewer (Mozilla's open-source pdf.js) as an overlay on your inbox: you stay in the thread, get a page counter, and press Esc to close. Nothing downloads, and the file never leaves your browser. Checked in real Gmail: Preview still works while the Acrobat extension is installed.

It also works in compose, reply and forward. There, Gmail downloads when you click an attachment chip; Preview lets you check the file without downloading.

Still stuck?

Note your Chrome version (chrome://version), whether it happens with every PDF or one, and whether Incognito changes anything. That is what Google's Gmail Help Community will ask first.

Gmail PDF preview not working? Fixes that actually work