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)

As mentioned, the Outlook App of Windows, running on the same PC where also Thunderbird is installed as standard email program, does not cause any problem. The PDF file could be correctly downloaded and open from Outlook. Similarly, on the smartphone the problem did not occur. the problem is inside Thunderbird. The computer is running Windows 10 22H2 64 bit, connected via WLAN , effective download speed as measured on the PC 14.5 Mbit/sec, upload speed 2.3 Mbit/sec, Ping 21 ms- The email account is xxx@outlook.com, IMAP The problem happened for two different emails received from different sources with different PDF attachments, in one case 1.2 MB (but Thunderbird is showing an attachment of 240 KB, email received on 2024.09.09) , in the other case 1.7 MB (email received on 2024.09.18) . in the latter case, at a later time, the email could subsequently open correctly the attachment, although I did not remember having changed anything in the meantime on my side. In all cases, the emails opened correctly in Thunderbird and the text of the email body was always correctly displayed, but instead of opening the attachment, a blue line was shown on the screen. After clicking on the attachment and having saved it on the desktop, the message from Adobe was popping up, indicating that the file was corrupted and could not be opened. I tried to recoved the first file with online recovery tools, the result was that some pages of the PDF were damaged. in the first case the installed Thunderbird version was 128.2.0, in the second case 128.2.1, I guess. My impression is that some data exchange between Thunderbird and the Microsoft server of Outlook.com was causing the problem. I wanted to highlight the issue because it may well be a generic issue.

(I noticed too late that you have many other postings on this subject)

Please post updates and replies in https://support.mozilla.org/en-US/questions/1468434