Do you mean : A) you are using 'Write' new message and attach pdf and then try to open the attachment from the Write window - before you have sent it ? OR B) you are using 'Write' new message and attach pdf, then save as a Draft. Locate saved Draft email and test opening of pdf attachment ? OR C) using 'Write' new message and attach pdf, send email and recipient cannot open pdf ?

How are are you choosing to auto open pdf document?

Check settings in Thunderbird:

• Settings > General
• scroll to 'Files & attachments'
• Content Type: Portable Document Format (PDF)
• What action is set?
• If 'Preview in Thunderbird' then pdf attachment gets opened in a new tab in Thunderbird.
• If 'Use Adobe Acrobat Reader' then pdf gets opened using the program you selected.

What do you have in your setup?

On Computer: what program have got set up as default to open pdf documents?