Thanks for your answer leo.
The alternatives listed here are basically either installing Quick View Plus 11(+) or the corresponding programs for the files you want to preview such as Microsoft office suite (which I don't use).
I don't need a standalone viewer and I don't really like non-portable software. I was using those libraries with Total Commander for all kinds of files and they work beautifully (even the 64bit ones). They also seem to be working perfectly under Directory Opus 32bits so there must be a bug with MultiView plugin for 64bits.
Since you say using directly the libraries with MultiView is considered legacy, does that mean it's not going to be fixed?
About the file formats I want it for, as I said before a bit of everything, most often Office files though...but I don't have MS Office in my home computer because I prefer Google docs for light writing and LaTeX for serious things so...
Really, the way it works with Opus 32bits and MultiView would be perfect if it worked on 64bit 