Gatekeeper refuses to allow **file** (not app) to open when nondefault app is selected

I have the same problem described here https://discussions.apple.com/thread/6709486

Looking around the internet it seems that this problem crops up not too infrequently and that moving forward from Yosemite didn't always solve the problem. The solutions I saw on the internet involved changing the default app for **all** files of the same filetype, disabling Gatekeeper or severely downgrading its protection or changing the file association on a file by file basis (requiring an admin password every time). Here's my workaround for opening some pdfs in Acrobat and most in Preview.

*Preview stays the default app

*Select (multiple) files to be opened in Acrobat (non-default app)

*Right-click ("open with" should show in contextual menu)

*Hold option while contextual menu is visible (do NOT hold option and right click at same time, must right-click first, SEPARATELY from option). "Open with" changes to "Always open with"

*Select Acrobat (non-default app) to always open the files with - you will have to enter admin password, but one entry suffices for all the files

*The files should open in Acrobat and also open in Preview (default app) without throwing off gatekeeper errors