The highly recommended file format by Issuu is PDF. However, you can also upload files in Word, PowerPoint, Google Docs, StarOffice, ePUB, and other formats.
If your sole purpose is to use Issuu to upload files and share them with others, you can do that even better via UPDF. UPDF can help you create PDFs on Windows or Mac from different file formats and then generate a shareable PDF link to share PDFs with others.