Start with the right checks.
One damaged download can fail even when the PDF reader works normally. First find out whether the problem affects one file or every PDF.
What to do next.
Confirm the sender or website is the one you intended to use.
Download a fresh copy from that trusted source. Save it somewhere you can find, such as Downloads.
Try opening a different trusted PDF. If that works, ask the sender to recreate or resend the problem file.
In Acrobat or Acrobat Reader, use Help > Check For Updates. If all trusted PDFs still fail, ask support to check the app installation.
Check the result.
The correct document opens and its expected pages are readable. Do not assume a file is safe just because it opens.
Stop and get help if…
The file requests an unexpected password, security tools block it, or the source cannot be verified. Keep protective settings enabled while seeking help.