I think I finally found a FIX for this problem.
* (1) Go into: Menu\Preferences\Security (Enhanced) and un-cecking the following:
Enable Protected Mode at startup
Run in AppContainer
Enable Enhanced Security
Close settings window & PDF file would open IMMEDIATELY, but only one time.
********** NOTE: Step 1 alone DID NOT FIX because "Enable Enhanced Security" kept defaulting back to being checked, causing me to have to do this ALMOST every time I opened a PDF from my email and even from my local drive (See step "2" below). Otherwise, took 30+ seconds to open. Got VERY used to the shortcut (Ctrl+K, Alt+E, Enter!)... after finally having enough of this -
* (2) STEP 2: Since Step 1 above didn't seem to retain its settings due to "Enable Enhanced Security" re-enabling itself every time - I think I FINALLY found a fix that works.
Go into same menu: Menu\Preferences\Security (Enhanced)
Click "View Windows Trusted Sites"
Select "Internet" (should be defult), then drag the "Security Level for this zone" to "Medium" setting.
(mine defaulted to Medium-High)
Click "Apply" and close the settings window.
This fixed problem for me - Enable Enhanced Security still reverts back to being selected, but on the Medium setting, all PDF's now open as soon as I double-click - even when app is not already open.