You can move documents between a Macintosh running System 6 and a Macintosh running System 7 without any problems. In most cases, your documents will not change their appearance. For example, if your document uses Times font in size 12, your document won't change.
An exception occurs when you have TrueType fonts installed and you have documents using numerous odd type sizes. Line endings for such documents may change slightly when you open the document under System 7. Also, the document's formatting may be slightly different. You may need to readjust your document's formatting to have the format for lines and paragraphs appear exactly as before. This happens because the substituted TrueType fonts are more accurate and true to the printed output you'll get. But if you want the document to look exactly like it did under System 6, just remove the TrueType fonts from your System file. They are the ones with the triple "A" in the icon which display three different sizes when you double-click on them.