This depends on the program. Most work with text, .txt files. Other programs create .odf, open document format files, whereas others use different types of formatting. Other formats include:
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Microsoft Word is a word processor and editor. When you save the file of a word document, it becomes a .doc. That is a file name corresponding to documents.
Rich Text Format (.rtf), Web Page (.htm or .html), or Comma Delimited (.csv), PDF, XPS, DOC, DOCX