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PDF - Wikipedia
What is a PDF? Portable Document Format | Adobe Acrobat
What is Portable Document Format (PDF)? - TechTarget
What Is a PDF File (and How Do I Open One)? - How-To Geek
What is a PDF? Portable Document Format | Adobe Acrobat
About the Portable Document Format – PDF Association
What is a PDF? A quick guide to its benefits & PDF tools - iLovePDF
What is PDF Format | DocFly blog
Basic Computer Skills: What is a PDF File? - GCFGlobal.org
PDF | Definition, History, & Facts | Britannica