[xubuntu-users] pdf file question

[truffle at blueyonder.co.uk]

hello.
forgive me if this is a silly question, i don't know how to fix this, or
even what to call it to ask for help.
i used to open pdf files without a problem until one day when i clicked on
one and it opened absolutely huge. only a small portion of the file was
visible on the screen and all of my toolbars disappeared so i couldn't
navigate around the document or to anywhere else.
now when i open any pdf file it opens in the same way and i have to shut
down the computer and start over again. is there a setting somewhere that
i need to adjust to correct this?
i am using xubuntu 6.06 and firefox 1.5.0.9
i don't know what program opens the pdf files.
thank you
a.m.