Thunderbird 102 is no longer supported it is so old, and here you are asking about something that was introduced in V91. https://www.thunderbird.net/en-GB/thunderbird/91.0/whatsnew/

I do not have access to such an old version to give you direct instructions, so I will hope that the ancient version you have is sufficiently like the current version that you can find the helper applications in settings to change your preferred helper application for PDF files.

The following image is what it looks like in version 1280 You of course will be selecting the PDF entry not the ZIP entry like I was in this image.

Thanks, Matt, for your help with my question. I've now found the Files & Attachments section in v102 -- one of the options for PDF files is Use Adobe Acrobat (default) and that must have been ticked (by default) in earlier versions from 91. Tbird, in its wisdom, changed that in my copy of ver 102, and I have now changed it back to the Adobe option as above.

While I'm grateful that you identified the settings element that I wanted, I was very surprised that, particularly as you are a Moderator, you chose to use such pejorative language answering my question. There are many Tbird users who, for good reasons (usually associated with add-ons that are not available in later versions), use an older version. I do hope you will treat future enquiries from users of older versions with a little more understanding and kindness.