KB User's Guide - Documents Tab - Pasting from Microsoft Office or Google Docs
You can paste Microsoft Office and Google Docs content directly into the KB editor (TinyMCE). When you paste, you will be asked if you would like to keep or remove formatting. It is recommended that you choose to Remove formatting. Please note that this only applies to extra formatting and styling; basic formatting, such as bolded text, will always be retained.
Copying and pasting from MS Word without removing formatting often pulls in a lot of unneccessary (and unwanted) formatting, including Microsoft-proprietary tags and attributes that could keep your text from displaying correctly in all browsers. It also retains styles specific to the application that was used to create the document.
For example, a document written in MS Word with the "Calibri" font will be pasted as that font if you choose to keep all original formatting. It may also have a different font size, line height, and/or custom margins. Because most of your documents are using the fonts, margins, etc. that were chosen for your entire site, your pasted document will look out-of-place.