There are free web only versions of Word, Excel and PowerPoint, but they're fairly limited. There's also Google Docs, which is online or has an accompanying Android or iOS app. If you must have something installed on your PC, then try OpenOffice. All of these options will be able to open and edited Microsoft files like docx, xls, ppt, rtf, txt, etc. but there sometimes can be some formatting problems. Also, macros stored inside a Word doc or Excel sheet might not work properly.