Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. User information is stored in folders of different types that include emails, calendar items, notes, contacts, and several other file formats. PST files are used for archiving emailing data offline that can be later loaded and viewed in various applications.
Portable Document Format (PDF) is a type of document created by Adobe back in 1990s. The purpose of this file format was to introduce a standard for representation of documents and other reference material in a format that is independent of application software, hardware as well as Operating System. PDF files can be opened in Adobe Acrobat Reader/Writer as well in most modern browsers like Chrome, Safari, Firefox via extensions/plug-ins.
You can also convert PST into many other file formats. Please see the complete list below.