- Check the file size. A PDF of a few bytes, or a download that never finished, is not a document. Download it again from the source.
- Drag the file into a Chrome or Edge tab. If it opens there and not on double-click, the default app is wrong. In Settings, Apps, Default apps, choose Edge or Chrome for
.pdf. - If no reader opens it, the file may be truncated or protected. Ask for another copy. Do not turn off antivirus to force it open.
A PDF from a stranger can be only an attachment. Opening it in the browser limits what a bad file can do. Do not run an .exe that came with it.