PDF (Portable Document Format) is a digital format for distributing documents created using a wide variety of applications (Word, Excel, Access, Quark, etc.) without the user needing to have a copy of these applications. To view PDF files, users need a copy of Adobe's free Acrobat Reader. To create PDF files requires a full version of Acrobat.
In a learning context, PDF files are most typically used to publish Word and desktop-published documents online for learners to download and print.