Hi, A friend created an InDesign document on an Apple Mac, sent it to me. I opened it on a PC running InDesign (64 bit) V9.0. I was able to make a lot of changes. When I sent the document back they were unable to open it. I don't understand how that is possible. Isn't the indd format defined such that both PC's and Mac's can work with the documents. Can anyone help?
It almost surely means that they don't have InDesign CC 9.0 (your version). They have an earlier version. Save it as an INDL file.