Are these attachments intact when accessed via webmail?

Yes, the attachment are viewable from webmail

I see you're running Windows 10. Which antivirus program are you using?

What type of attachment is it? And what is the exact message that is displayed when trying to open it with Thunderbird?

I see you're running Windows 10. Which antivirus program are you using?

Windows 10 with Trellix Endpoint. Just FYI, I've removed Trellix before but the issue persist

What type of attachment is it? And what is the exact message that is displayed when trying to open it with Thunderbird?

In this case we intend to receive a PDF file. Attachment size decreases from 106 KB to 94.5KB once the email is downloaded to Thunderbird. Its happen to any other file format such as xlsx & docx

The TB PDF viewer show blank when open the attachment.

I'll get Error "The file is damaged and could not be repaired" when open the attachment file using Acrobat Reader

Something is evidently interfering with the download and number one culprit is usually security software. My installations of Thunderbird 115.6.1 on multiple PCs are not facing this issue, so we can rule out a bug in the product's core code. Can you recall what transpired right before this started happening, say a Thunderbird upgrade? Create a new Thunderbird profile and see if this persists. For more information, see Profile Manager - Create and remove Thunderbird profiles. This will rule out if it's an issue of profile corruption or not.