I updated versions today. Both the signing apps have the same error across IOS devices.
The alert dialog shows the message "This form requires Adobe Acrobat Reader DC for Mac or Windows."
Your form is a special type of form called "dynamic XFA form". This particular type of PDF form is not supported by the mobile version of Acrobat Reader (for iOS, Android, or Windows Phone).
As far as I know, there are no PDF viewers that support XFA forms on mobile platforms.
The alert dialog suggests that you use the desktop version of Acrobat Reader DC (Document Cloud, not Creative Cloud) on your Mac or Windows computer. No paid subscription or sign-in is necessary to view XFA forms using the desktop Acrobat Reader app.
Sorry for the inconvenience.
That was a PDF sent to me. I needed to be able to open the PDF, put a digital signature on it and send it back. Neither of the available apps designed for signatures would open it either. I find the fact that adobe would create a platform that could not be opened on mobile devices either neglectful or dimwitted. I am not satisfied with the answer and feel Adobe has not met the level of service to a customer and subscriber. To correct the issue, any PDF that can be opened on the desktop should able to be opened mobile.
As @rddempsey mentioned, this is a pretty big flaw for the PDF service... I wonder if the user who creates XFA is warned of the fact that such a form cannot be opened on mobile devices. I could see a whole heap of problems arising out of this especially in the case where documents/contracts need to be filled or signed quickly and sent. The hopes would be at least that it can open the file without all the form filling functions, just the ability to print. I understand this would have a lot to do with format and structure of an XFA form, but the consideration would be to get rid of such a PDF format and implement something that works across all devices. Or at least discourage users from creating such forms by informing them of the deficiency. I encountered this issue trying to fill out a complaint form against a medical practitioner in my state... this is something that arguably should not be implemented in such a way. Just considering the user case for older generations 60+ who may not have a computer but fill out paper forms or use a device. It really creates a really bad experience with Adobe products. I hope you guys work something out...