Since my name was mentioned, I'll post here but it's only to add my support to Twix123's suggestion. There are lots of pieces of software which do the job. I've used CutePDF in the past (and I can't recall any problems with it, except that you have to install GhostScript first) but I've got doPDF at the moment and I'm really happy with it.
CutePDF
http://www.cutepdf.com/Products/CutePDF/writer.asp
doPDF
http://www.dopdf.com/
As Twix123 says, to save a document as a PDF you need to click on 'Print' (which seems rather odd at first, since you're not actually printing anything!). Then change the printer, from whatever you normally print with, to 'CutePDF' or 'doPDF' as appropriate, using the drop-down menu, and click 'Print'. You'll be asked to choose a destination folder (such as 'My Documents') and to click 'OK'. That's all there is to it. (As far as I recall, CutePDF stops there. doPDF also opens the saved file in your PDF reader, such as Adobe Reader, so that you can see how it looks).
Applications such as CutePDF and doPDF work with
any program that you can print from. If you're designing posters, you might be working with Microsoft Word or (far better) a proper DTP program, such as Microsoft Publisher. If you've not got a DTP program, this freebie is really good:
http://www.serif.com/desktop-publishing-software/?mc=FSSPAGEPLUS