PDF (Portable Document Format) is a filetype that represents a document. The PDF format was originally a closed format developed by Adobe, but became an open standard in 2008.
PDF is fairly widely used for the storage and transfer of documents that are principally designed to be viewed and archived (e.g. invoices, quotes, documentation). However, the PDF standard does support editable fields, so PDF documents are sometimes seen acting as forms for the request and submission of data. The inclusion and growth of such interactive functionality has led to some fairly serious security vulnerabilities in PDF renderers, where a malformed PDF document can be used to attack a system with a vulnerable PDF reader.
Systems that automatically generate PDFs from a set of data are quite common, although developing this functionality and interacting with various libraries that allow this can be quite challenging. Similarly, systems that automatically ingest and parse PDF documents can be difficult to develop, given the large and varied PDF specification.
More details about the format can be found over at Adobe.