That means your Office preview handlers are broken, unless you're not using MSOffice or are using a very ancient version which predates preview handlers (Office 2003 or 2006 were the first versions with them, from memory. Not sure which.)
Reinstallign MSOffice may fix them. Get them working in File Explorer first, then worry about Opus.
Opus is probably falling back on using Office itself, not the Office preview handlers, to view the files, which will always be slower, and also does not work as well. (This fallback is disabled by default in newer versions of Opus.)
For PDF, you may need to install a PDF preview handler such as Adobe Reader. As with Office, check that things are working in File Explorer for that format.
TXT files do not open in Office so I'm assuming you mentioned those in error? They should open instantly in the built-in Opus text viewer.