is there any error? Or what prevents you to open that file?

So far it has been only PDF files. When I single or double left click the file, nothing happens. No error. I can right click the file which opens a menu, the top box being "Open" and then clicking the "Open" box does nothing. Clicking "Remove file" lower in the list works.

Note that this just started a few weeks ago. For years I had been able to open attached PDF files by just left clicking them. I have no problem with opening clicked files on my work computer.

On right click - should be open - and after that open with... - can you select something else there? If yes try to select a browser and with check-box for "do this from now on". After that once again open - open with - select your pdf viewer with same check-box. If no - can you add a screenshot with last option which appear before "nothing happen"?

Options/General/Files & Attachments, delete the entry for pdf files (Delete Action from the Action drop down), restart TB, then open a pdf attachment, don't choose the helper app from the list, but Browse... to the exe for the app that opens pdf files.