A family of Microsoft word processing software products for creating web, email, and print documents.
Most commercial (non-open-source) programs have a native file format, which is what you refer to as "custom". Acrobat's is .pdf, Word's are .doc, .docx and variants for templates, etc. These are usually formats that can deliver every feature of which the program is capable. Other formats that are not specific to a particular program, like .txt, may have reduced capabilities that make more generic documents.