Save as the older format instead of the newer one (.doc/.ppt/.xls)? newer format that has x behind (.docx/.pptx/.xlsx) are actually zip files to make them smaller, and some apps sometimes have issues figuring out whether its a zip file or not. Its a whatsapp issue.

Just find other ways to send your document (email,cloud,google docs etc) if you want to keep it in the newer format for some reason.