[SOLVED] reading a pdf document

I get weekly invoices from a company and they are sent as attachments in pdf format.
When i click on these the "open" option is word (default) , when i go through other options no pdf program such as adobe appears in the list.
The only way i can read the pdf attachments is to open something else that just happens to be in pdf format , if i then leave that item open i can read the invoices.

What can i do so that i can just click on the pdf attachment and read them straight away.

Dont know if this makes any difference but i am using the thunderbird email program.