Which file format enables users to share a document on different systems?

It all depends on what you mean by 'document.' Some use that term for any file but for computer people, a document is a file that can be like a book or text file with picture or no pictures. If you want to share text and work with another person, the best solution might be using the .rtf format, which stands for 'Rich Text Format.' It is a generic type of text file that allows you to make bolds and italics and such so that you can leave notes to a colleague. If the colleague is doing the work and that person wants corrections from you, the best format may be .pdf, which stands for 'Portable Document Format.' This format allows you to mark it up, like a teacher might, with notes, circles, x outs... etc. If you are on a Mac, the 'Preview' app can do the mark up. If you are on a PC, then you must download and load the free PDF Reader.
Rtf and pdf work on all computer OS's including iOS and Android.