PDF or Portable Document Format is described by Wikipedia as follows:
Portable Document Format (PDF) is a file format used to represent documents in a manner independent of application software, hardware, and operating systems. Each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, graphics, and other information needed to display it.By enabling this feature, FamilySearch is encouraging users to submit copies of documents they may have in their personal collections that act as source material for the user's ancestors in Family Tree. From previous announcements, it is likely that in the near future they will also provide a way for users to link the documents to a specific source reference.
I suggest that you carefully read the Documents Agreement that appears when you go to upload a document to FamilySearch.org and the Submission Agreement also. You should be familiar with both these policy statements.
I have been trying out this new feature and it seems to work very well.