Some file types, like standard PDFs, will upload and immediately be able to be displayed and editable. Others, like .doc, .xls or .png can't be displayed ("previewed") and edited right away: they need to be made into PDFs first.
Once you've uploaded a non-PDF file, clicking on it will give you these options:
Clone & Convert: Opens the file in Remine Docs and will create a PDF of the file that can later be marked up and signed, etc. The original file that you uploaded will remain unaffected.
Download: Save a copy of the original file to your computer/device.
There are some file types that cannot be rendered into a PDF; most commonly, .zip files. These files can be uploaded and stored in Remine Docs+ but cannot be opened or edited/signed.