Ok, I have a workaround.
First, a preface for the Adobe folks: I had been going to my banking websites (I have more than one) and clicking on their button to download my monthly statement in PDF format. My IE11 browser always offers me the choice to Open, or Save, or Save As. Habitually, each month I select Open so I can first view the document then, in the Adobe reader itself, I click File then Save As. Once I click the final button to actually save it, is when the Error 21 pop-up appears. I noticed that I have no problem when doing the same thing today from another online bank, or from online websites that are not banks. I felt I had isolated the problem to just one certain online bank.
So I called that bank today asking if anyone else had called them to report this problem. Apparently I was not the first one to call because they brought a lady to the phone who acknowledged I was having Error 21 and asked me if the pop-up I saw said their bank name or if it said Adobe. I replied Adobe. She then told me to do this:
The workaround: when my browser offers me the choice to Open, or Save, or Save As, I am to select Save (not Open and not Save As) and let the PDF save itself to my Downloads folder. I did this and it saved. Then, I went to my Downloads folder, found it there under some generic filename they chose (Estatement.pdf) and I opened it and closed it with no errors. So far, so good. I even had her wait while I moved it to the folder where I wanted it to be, and even renamed it, and it's there and saved with no errors. Opens and closes ok. Yay.
She said it is an Adobe problem and that Adobe is working on it. I asked her how did she know that, was the bank in dialogue with Adobe? She said no, they just went to the Adobe site (probably this thread!) and saw where Adobe said they were working on it! She said it is happening because the file is encrypted. I asked her why then does this work ok for my other bank, surely they encrypt as well. Here's where I should have written down exactly what she said. She said something like the other bank probably used different encryption like SSL (?) but this bank changed away from that because it got hacked. I can't recall what type of encryption she said they used.
Anyway, that workaround worked for me. Hope this helps.
