I tried unchecking the Smart quotes option in both the MAC system preferences system preferences -> keyboard -> text tab untick the use smart quotes and dashes option and in textedit app preferences at textedit -> preferences -> uncheck smart quotes and dashes in rich text documents only option
But the issue was still not resolved for me. When I ...
The following workaround does at least provide a fixed user-settable zoom (TextEdit 1.13, OS X 10.13.6 [High Sierra]).
Create and save a blank TextEdit doc (maybe make it Stationery via
Open the doc in TextEdit (don't use "Open Recent") and
select the Option button to "ignore rich text commands". (Or, open it
in some text ...
If the data is still correct on other computers, your local Dropbox copy might have gotten corrupt for whatever reason.
Ensure that all newly created files are synced back (make a local backup if necessary)
Log out of Dropbox on the MBA
Delete the local Dropbox folder
Log into Dropbox again and let it sync the uncorrupted files onto your Mac