Problems with Thunderbird

[genda]

Hello,
I have been having  problems with Thunderbird lately but what's been bothering me lately is the fact that I download PDF reader but every time I click on a attachment, it only give me the options to browse and when I did, the options of PDF reader is not there even though that I downloaded it on my PC. I don't even know what to do! I have try options and attachment but doesn't have that options.
I know it's probably a simple fix but I don't know where to go. Please help