Generally, 'helper applications' are defined to the OS, so when thunderbird attempts to open the PDF, the OS's default app is used. My suggestion is to check the OS default for PDF.

Thankyou but this doesn't solve the problem. My default app for .pdf files is correct. The problem occurs when I create a pdf drawing, invoice, etc. and attach it. I typically like to attach the pdf and check it one more time before sending. Thunderbird will not allow me to open the pdf once attached to the outgoing email. This began when upgrading to 15.10.0.

Thank you.

I don't have this problem in Thunderbird 115.10.1 on ubuntu I can see the PDF before sending it. It opens a new tab with the path to the file and "?type=application/pdf" at the end of it.

Maybe this question helps? [to open PDF files in Acrobat Reader DC instead of in a Thunderbird tab]

> 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.

Probably helps to knows the particulars? what is your default app what OS are you using?