Did you check where you found the original document? Or do you remember under what name you saved the document?
Document Viewer is usually used to read documents which come from the Internet (check your Downloads folder) or from clicking on a file in File Manager.
LMDE Mate 64-bit, LM17 Mate 64-bit
Debian Mate 64-bit, Xubuntu 14.10 64-bit, Ubuntu-Mate 14.04 64-bit, Antergos Xfce 64-bit, PCLinuxOS Mate 64-bit