Adobe acrobat comes in two parts: one part (the Distiller) creates Portable Document Format files (".pdf"), the other (the Reader) enables people to open and view these files. The distiller is commercial software but the reader is free for anyone to download and use.
The idea is that companies can generate a "pdf" file from virtually any application and anybody can then access it using the free reader. This has been especially successful on the internet as people will be using a variety of different types of computers (never mind software), but they can all download and run the Acrobat Reader.
Many companies now put the manuals for their products on the web in pdf format for people to download. In addition the UK government uses the format to publish reports etc on its website.