If the PDF Content Type is not shown in the Preferences then it is likely you have not set it up. Your image shows you have not set up anything.

Right click on pdf attachment and select 'Open' Select 'Open With' and choose from the drop down options. PDF typically uses 'Adobe Acrobat DC' or 'okular' - but these programs do need to be installed on your computer. However, Thunderbird version 91* can now open PDF in a Thunderbird tab using the option 'Preview in Thunderbird'. select the checkbox 'Do this automatically for files like this from now on' and it get saves into the Preferences 'Files & Attachments' location. click on 'OK'

This should then add that PDF document option into Preferences > General > Files & Attachments.

From that place you can now reselect the 'Action' using drop down options if you want to change the action. It also means when you right click on pdf attachment it should open using whatever you have set. You should not need to set it up again. It should use your prefered option automatically.

Hi. We had the Adobe reader installed a long time ago, and the problems didn't come until the arrival of the 91 Thunderbird. It is likely to be a problem in the current user profile, as we see all the possibilities when we set up the new Thunderbird profile and can influence the behavior with the PDF. Unfortunately, the user has a lot of mail in the current profile, and converting it would be a big problem.

Do we have any other way to set the behavior when opening PDF attachments? In the Config Editor, for example? All I'd have to do is turn off the Thunderbird-integrated PDF viewer.

Thanks so much for answering

re: when we set up the new Thunderbird profile

If you create a new profile - perhaps a new person is joining business - I do not know why you need a new profile. In a new profile, there are no settings - hence why the image you posted has nothing set up. You have to set it up to choose whatever you want. You have to follow the same procedure to set up the desired option for each type of attachment.

re: All I'd have to do is turn off the Thunderbird-integrated PDF viewer.

You already have Adobe Reader installed so you need to select it. I've already told you how to do it. It is not a 'Config Editor' settings, it is a simple Preferences setting. It is the same method you would use to set up the action for any Attachment. You only have to do this once to set up for PDF.

I've added some images to help explain what to do. Look at the images I've posted.

First image shows I have an email with attachment which is a PDF. Right click on attachment and choose 'Open'

You should get another window which asks 'open with'. Do you get that window or not ?

You choose 'Other' from the list of drop down options and it opens a small window showing options. Select the 'Adobe' option and click on OK Then select the small checkbox to always use that option. Finally click on OK.

Then that option is stored in Thunderbird. It is stored in the Preferences > General > Files & Attachments See second image.

You need to explain why you cannot do what I have shown in the images.