Can Only Open Adobe Acrobat Reader with "Enable Protected Mode at startup" Box Unchecked.

I have already asked this question in another thread but it sort of got lost in translation.  I thought I may get better luck by starting a new thread so here goes.

To start with, up until recently, I've had no problem opening Adobe Acrobat Reader PDFs in Windows 10 on my laptop. 

The version I have is Adobe Acrobat Reader DC Continuous Release | Version 2018.011.20055 (The Latest Version).

I then suddenly could not open any PDF or they were very hit and miss as to whether they would open or not when restarting the computer.  When I say, I could not open them, I mean just that.  There was no error message or anything like that when trying to open them.  They just would not open when double-clicking on them or right clicking them and then clicking open.

Because I used to be able to open PDFs without any problem, my thinking was that the latest Microsoft update may have caused this problem.  I may be wrong but I cannot think of any other reason why Adobe PDFs would have suddenly stopped opening.

After some investigating on Google, I stumbled across the solution.  When I did eventually, manage to open Adobe from the start menu, this is what I had to do.

Once opened, I then clicked on Edit > Preferences > Security (Enhanced).

At the top of the page then says, "Sandbox Protections."  Below that there is a small box that says, "Enable Protected Mode at startup."  This box is already checked.  If I uncheck it and then click "OK," then any PDF will open and they will open every time without any problems.

I would like advice from an expert in here please; if I leave that box unchecked permanently, am I unprotected in some way or is it safe to leave it unchecked?

If I am unprotected by leaving it unchecked, can somebody give me the correct course of action on what to do so that I can continue opening PDFs safely?

Really appreciate any help.

Andy