Edit PDF in sharepoint online

I just set up an entire workflow for staff and important committee members in which the committee members, who are external users, would mark up a fillable PDF with comments in comment boxes I created as fillable fields. I just got an email back from one of them saying his comments disappeared after he filled them in. The entire point of this was to make document review and commenting as easy as possible with as few clicks as possible. Requesting a committee of six people either use internet explorer (you overestimate how many people even know what the word 'browser' means) and follow additional steps, or download and reupload (bad, bad, bad practice in any situation) the PDF is not even under consideration. It's humiliating that I did a training for everyone, created documentation, and then come to find out that a feature that should be there isn't. 

(While MS is at it, please make fillable PDFs appear in their fillable form upon clicking on the file name in SharePoint - as it is, you have to hover over the file, click the vertical dots > Open > Open in browser for the file to even appear in its fillable form - which as previously detailed is a bait and switch anyway, since if you fill it out, it won't save)