Even in well-funded nonprofits, not everybody has access to the version of Adobe Acrobat that allows you to make PDF files from Word and Excel documents. Using a PDF (which stands for Portable Document Format) is often more convenient -- a PDF with images will be smaller in file size than a Word document, and thus easier to send via email. And even though some folks have the software necessary to edit PDFs, a PDF is less likely to suffer at the hands of overeager editors than is a Word document. There's just something much more final about it.
Lifehacker, an incredibly popular blog on productivity and "helpful hints" (read more about the editor here) recommends ExpressPDF, a free Web-based service. If you have experience, good or bad, with ExpressPDF or the similar services popping up (see the responses in Lifehacker's comments section), please share your thoughts here. (Note that ExpressPDF is not totally secure so find another solution for sensitive documents.)