Turn off Track Changes.
The marking and history of changes will remain in the document but future changes will not be marked.
Converting to pdf and then opening in Word is a horrible practice. Doing this will give you documents that are very difficult to edit and extremely complex under the hood. You could, if you need a record of the changes, simply save as a pdf as a copy. Then, in your original turn off the Tracked Changes and Accept All Changes to remove the markup, if you wish.
You can also save versions of your document. Here is a free Add-In to make this easy.
- Save Numbered Versions by Graham Mayor, MVP.
If you do that and later need markup, you can use the Compare or Combine features under the Review tab to produce the markup.