PDFs are printable download format documents or printer friendly pages. Many websites have PDFs containing all kinds of information, such as maps, brochures, layouts, lyrics, drawings, menus, recipes, whitepapers, etc.

Printer friendly pages are often exact duplicates of non-printer friendly html counterparts. Therefore, they can be detected as duplicate content and filtered out of the index.

Sometimes, a PDF will appear in the Search Engine Results Pages (SERPs) and the html page does not. The web master should choose which version - html or printer friendly  - of the page they prefer to have indexed and put no-index tags on the PDF.

Other PDFs contain unique and valuable content. Allow these to be indexed and optimize them for the Search Engines.