The point of Wordpad is that it is a simple document editor that has been included (free) with Windows since the 1990's. It has more word processing features than NotePad, an even older "text" editor program. So if you can't use Office for some reason, you
can fall back to WordPad.
The Word files are not "converted" to WordPad. The problem is that for some reason WIndows has been told to use WordPad to open DOCX files. That is the Windows Default File Association.
As suggested, you can set just the one file type using the "Set defaults by App" tool, but I strongly prefer using the Office Repair option, if one file association is off, there is a good chance others are also. So by doing an Office Repair, you reset ALL
file types used by all Office programs in one step. Doing a repair should have no negative effects on anything in your Office experience.
Repair Office 365 / 2019 / 2016 / 2013 / 2010
https://account.microsoft.com/
Here are some similar articles describing how to do Office Repairs for various versions of Office and Windows. Pick the one that easiest for you to understand
Repair Office 365 / 2019-2010 on Win 10-Vista
https://support.office.com/en-US/Article/Repair-an-Office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b
Repair Office 2013/2016 in Pictures
(My WIKI)
http://answers.microsoft.com/en-us/office/wiki/office\_365hp-office\_install/repair-office-365-in-pictures/f09d64b7-cfcc-4d07-be0b-0c429682d01a
Setup command-line options reference for Office 2013 including repair
http://technet.microsoft.com/en-us/library/cc178956.aspx
Alternate method of Repair- Fixing Word
http://wordribbon.tips.net/T001232\_Fixing\_Word.html
Need to make sure that Word is installed correctly from the original installation CDs? You can do it by using a command built directly into Word.