Glad you worked it out.
all that shows is Gimp and Open with Other Program....
when I click on Other, it takes me to my Home folder.
For finding programs to select when they're not listed, generally you can find them in the /usr/bin directory (not sure if this is the case with acrobat reader, known in Linux as acroread, or not). Besides acrobat reader, there's a program called ePDFviewer (located in the menu in the section "Office", with the executable at /usr/bin/epdfview) that can be used.
But yes, I too had the experience of Thunar automatically opening pdf files with Gimp. I don't know if the developers could change that to have Thunar recognize ePDFviewer as the appropriate program right from the get go, but if they could that would certainly be a good idea.