How to instruct Thunderbird to use Adobe Reader to open .pdf files. Right click on the attachment and choose 'open' a new window opens choose 'Open with' and select 'Adobe Reader' from the drop down list. select to 'Do this automatically for files like this from now on' click on OK This will now show up here: Tools > Options > Attachments > Incoming tab or 3bar menu icon > Options > Options > Attachments > Incoming tab under content Type it will show 'adobe acrobat document (.pdf) and to the right it should show the 'Action' to use adobe reader. If you want Thunderbird to always ask where to save files, you can select this option or choose where to save files to and use Browse button to locate a folder. click on OK