This type has been deprecated in favor of one of it's alternatives. While you should likely still support it, it should not be your first choice if assigning a MIME type.
Instead please use application/pdf

Portable Document Format (PDF) is a document file format originally from Adobe, based on PostScript. It has many subsets.

Used in file types
  • .pdf
This type may also appear as
  • application/pdf
Further reading
  1. Let's Solve the File Format Problem!