Technically there is no "desktop app," but if you mean the web interface, then yes -- if your documents are not already PDFs, you would need to convert them to PDFs to view them there.
The web interface is more geared toward entering and editing data and has limited capabilities to view or interact with your documents.
But the Android app, also, can only view PDF files. iOS is the only one of the three platforms that has built-in capabilities for viewing a variety of file formats.