Perhaps your antivirus program is corrupting the download. It has happened frequently in the past.

I do not think the problem can be originated from the antivirus program (in my case Defender) . The same email once downloaded from the Outlook App running on the same PC does not have the problem, the PDF attachment always opens correctly. It cannot even be the internet connection, as the problem never comes up in Outlook. So the issue must be in Thunderbird, either something went wrong in the Account settings (@outlook.com) or in the recent updates. I have been using Thunderbird for about ten years and this problem has only occurred in the last month.

If you want folk to stop guessing, perhaps offer some information. The primary cause of attachment corruption historically has been antivirus scanners because they are often really bad at what they do.

Now I know you have Defender, it probably is not an issue because it does not scan email attachments before they are opened. SO; What mail provider are you getting your mail through? Is it messages from the same source that are always affected? Do they open in Thunderbird? (they should by default)