[nfbcs] pdf help please!!!!

[Alexa]

I use the following: wind7, Microsoft office 2010.
Recently I was downloading a pdf file. As usual I got a dialog box asking
where I would like to open the file? Unfortunately I've selected MICROSOFT
WORD OFFICE 2010, and checked save this downloading file as default to word.
When I open word after the download process, I can't see nor read the pdf
file, it seem like its invisable. QUESTION, how can I undo this? How can I
change downloading pdf files back to pdf as a default? For instance, I was
downloading an application which is in pdf file, dialog box ask, where to
save or open? I made wrong selections I selected msword 2010 and I wrongly
selected do this as a default. Now I can't open nor read pdf files. Please
help.