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Different PDF File Types Explained | Adobe
PDF - Wikipedia
8 Types of PDF Standards – Each Serves a Unique Purpose - Marco
Types of PDF Files: Everything you Need to Know | DocFly blog
PDF Standards: Specifications of PDF/A, PDF/UA and More | ABBYY
PDF Evolution: ISO Standards, Subsets, Versions and Types
PDF standards explained: with a focus on the newest
What is a PDF? Portable Document Format | Adobe Acrobat
1. Introduction - PDF Explained [Book] - O'Reilly Media