There was an error opening this document. The file cannot be accessed by the system.

I helped someone with this issue recently that had trouble opening files on a network drive- and in our case it was a problem with the total pathname length (folders + filename) exceeding a limit of 256 total characters.  In this case she had a lot of folders categorizing her work and had a very lengthy filename.  When I reduced the filename under this limit the file opened normally.

This is a late response I know, but I just want to help whoever can be helped because I had the same issue and it drove me crazy so I know if it drove me crazy and I'm looking at all the responses. It drove others crazy too, but my resolution actually ended up being not to go through the creative cloud, Adobe acrobat app at all, I was able to open up my file through Adobe acrobat online and just sign into my creative cloud online then I was able to open up my document that I couldn't open up. I kept getting the error that I couldn't open it up. The file was nowhere to be found , but thank God, I was able to open it up online through the desktop I hope this helps somebody because I was certainly looking for help.

I have solved the issue. Hopefully this can help someone else with the same issue. 

When you are in the folder containing the documents you need (it can even be the first folder you click into) click file, then options, and then click the button at the bottom "restore defaults". I was then able to open my files. 

Solved! I had the same issue with a pdf on dropbox. It was the security settings. File>Preferences>Security (enhanced), and then I added my entire dropbox folder to the trusted file path. There may be a more precise, secure way to achieve the same result, but this way worked. Hope it helps someone else!

I ended up searching for this issue resolution outside the Adobe Support Community and found a correction.  I meant to come back here and tell everyone what it was but I forgot until I just got an email that someone else is having the same issue and added to the string.  I didn't see the exact link I came across but Essentially, the correction (and there are tons of step by step replies if you just go to Google and type in the error and not focus on Adobe forums) was as simple as resetting the default file settings.  Thereare more involved step by step instructions to correct the issue--the problem stems from a protected mode being enabled on startup.  

Same issue for me, and I'm seeing that these posts are all very recent. Misery loves company, and contemporaneous company is even better. Yesterday, I spoke with an Adobe support person. The Acrobat product I was using (quite successfuly for years) was Acrobat X Pro, but they no longer support that version. The support person offered me a discounted version of the online product, Creative Cloud Desktop. I agreed because it was affordable but I was skeptical that it would resolve the problem, which it didn't. Knowing that was a possibility, the support person assured me that because I bought the newer product, they would support me in further trying to resolve the issue. I have a call scheduled with a tech tomorrow morning, so hopefully they can fix this. I will post here again with the outcome.

Are these files saved locally, or on a network drive, or on the cloud? If they are not saved locally, try dragging them to the desktop and then opening them.

I've also tried signing into OneDrive...I'm still getting the same error.