Preferences/General/Files & Attachments, click the Action drop-down to select a different app (see picture).

In Thunderbird 91.0, there isn't a Files & Attachments section. It it's there is, please tell me where it is, in 91.0.

I've attached a screen shot of the Preferences/General tab from Thunderbird 91.0.

Your Preferences tab is severely truncated. Did you run an old profile in 91 or create a new profile with 91? Try running in safe mode (hold Shift when launching TB) and see if the full tab appears. If it does, launch again in safe mode (now called troubleshoot mode), check 'Reset toolbars & controls', 'Make changes & restart'.

I followed your instructions but it didn't work. It did reset my toolbars, but under "Preferences", there's still no "Files and Attachments".

I used an existing profile.

I read in the release notes that PDF files will now open in a Thunderbird tab instead of a third-party app. Opening in a tab is not for everybody, so I'm hoping that they'll add back in the flexibility to the end user to select how we want files and attachments handled.

Attached are 2 pictures that show the items in Preferences/General that continue where your picture ends. You can create a new profile from Help/More Troubleshooting, about:profiles, and see if you get the full tab. I always recommend creating a new profile when moving between major releases, just to avoid issues like this.

If I make a new profile, what will happen to all of my folders and mail?

The old profile is untouched and can be launched from about:profiles; the local mail can be copied from the old to the new:

https://support.mozilla.org/en-US/questions/1346590#answer-1434240

I am using Thunderbird 91 on Ubuntu 21. With the Thunderbird addon "Thunderbird Conversations" with pdf help enabled

I too find the management of attached pdfs in 91 to be difficult. First, the pdf/attachment icon is not always in the preview pane.

The pdf icon is at the bottom of the document, as is the forward option, which is not helpful for me.

The default of opening the pdf into a Thunderbird tab does not allow an option to print. Again not good for me.

In Preferences/General, I do have the "Files and Attachments" section, however there are only two types of files listed and there is not an option to add additional file types.

Maybe as sfhowes notes using an old profile with a major upgrade is the culprit. Preferably a more user-friendly method of updating the profile is necessary or more documentation from Mozilla on how to create a new profile and move email over would be helpful. Unfortunately with multiple accounts and more than a dozen email addresses, adding them all back is a bit painful for me.

My solution for an alternate pdf viewer is Preferences/General/Config Editor search for pdf change the following setting from true to false "browser.pdf.launchDefaultEdgeAsApp"

Now clicking on the magnifying glass next to the pdf icon I have the traditional "what do you want to do this" dialogue box, allowing me to open it with the basic pdf viewer which provides a print option.

cheers, jeff

I did what Jeffmozilla suggested with the config editor, but pdfs still open in a tab for me. But I dont see any magnifying glass or pdf icon. Where should I look for this icon?

I figured out a work around (for my ubuntu 18.04 at least). Let thunderbird open the tab. Go the tab and click on the download icon. That will give you the standard dialog, which allows you to open the pdf with acroread among other options.

I agree with others above that the user should have the ability to use any helpers for pdfs like with other attachments. Hopefully they fix this.