PDFs are not good for . . .
Web Content
When your visitor clicks a link on your page, they expect to be directly connected to another page. But in some cases, when you click a link, a PDF is deposited into your download folder. This is not a good use of a web link because (1) the download is a surprise, and (2) some visitors will miss it altogether.
If your link leads to a PDF, it’s best to announce it and to provide a link to the free Adobe Acrobat reader.
Articles
When I’m expecting an HTML pageful of information, a PDF is the last thing I want. It has to be read in another program, and it takes up space on my hard drive. If you feel that your information is truly worth keeping or printing, offer an optional PDF download.
Specific Searching
The Google bots will search and index PDF files on your site. But just because they can be indexed doesn’t make them a great choice for content. Any topic search will return it’s share of PDFs complete with bolded keyword references and a link. But the link downloads the entire PDF!
Conclusion
PDFs are best suited to documents that are static and unchanging. Forms, legal documents and graphics files translate to PDF very well, but the format is less desirable for interactive and linked content that wants to be easily searchable and usable.