PDFs should be used and only used when the following apply:
IF
1. The document will be printed.
For example the document will be used for several people to sign at a meeting or this document is created to go into a paper system.
OR
2. The document is likely to be saved for reference on a computer.
For example, in addition to being accessed on the web or by email and the user will be likely to store the document. A signed contract.
AND
3. The document’s integrity is important.
For example that it has been designed for its look and feel and this needs to be consistent throughout its use. Or if it is important the document can’t easily be changed. A Journal article.
NEVER
Use a pdf on other occasions.
Forms are better in a Word file so people can edit them and return them or the form can be online in html. Information that is primarily on the web should be in html. In cases where use is mixed provide both a pdf and other formats. For ebooks consider providing epub, mobi, Kindle, Nook or plain text versions.
Later Consider Apple Author versions