Could you provide us more details about the Android version and the device that you are using? Have you placed some restrictions on the pdf form files that you have created? Would it be possible for you to share one such pdf file with us at firstname.lastname@example.org so that we may confirm the issue?
Android 4.4 thru 5.0 decided you CAN NOT edit anything on a SD card. I to lost all that when I updated from 4.2 to 4.4. I believe 5.1 restored the SD card file modifying capability. BUT, the new & improved Adobe DC seems to have thrown another wrench in the file access issue. There is NO DOCUMENT TAB, just a recent tab & a "CLOUD" tab. And, since my old PDF's were not viewed recently in the NEW Adobe DC, they were not listed, and I have not yet be able to access or find them on my tablet. All PDF's are not displayed by default now. Even the new file I did find, there seems to be no option to rename it. A complete revision is not a good thing for small business, or anyone - thanks for nothin' Adobe....
Problem has been resolved. Due to my cloud based program "Smartsheet" the files must be downloaded to the tablet then opened with Adobe DC. The option only appears if you long hold / click the file in smartsheet. Since this wasn't necessary before the update it made the file impossible to edit.
Adobe support was great and helped me figure this out issue throughout the night. This is a temporary work around but the issue should be resolved in the near future.