Are you using an IMAP account ? Do you normally download the full email or just headers (thus downloading as required to cache)? Do you know the type of download can be set up on a per folder basis ?

Check this: the folder which contains the email with pdf attachment - is it set up as available offline?

• Right click on folder and select 'Properties'
• Click on 'Synchronisation' tab
• Is the checkbox 'Select this folder for offline use' selected ?
• If no, then select that checkbox and then click on 'Download Now'
• click on 'OK'

Wait a few moments whilst that folder gets fully synchronised meaning full copies of emails get downloaded instead of just headers.

The file might be corrupted if there's an antivirus app scanning attachments. In general, the TB profile folder should be excluded from AV scanning (which doesn't affect the real-time background scanning).

I am having the identical problem as the OP, using my MacBook with Thunderbird 102.6.1. Having used TB for many years, PDF's always opened without issue, this problem pop up maybe a couple of months ago. A tab opens up in TB but it does not display anything, it's blank. If I download the file to my computer, I get the following error message, "It may be damaged or use a file format that Preview doesn’t recognize." Once downloaded I have try to open the file with several programs, each produces the same error.

If I go to the email that has the attachment on my iPhone, the file opens perfectly. If I open the email in my browser instead of TB, the file is fine and I am able to print it. So it appears that it is an issue with TB.

Try changing a preference that affects how TB downloads files:

https://support.mozilla.org/en-US/questions/1404538