The iPad doesn't use the full desktop version of Adobe Acrobat Pro DC nor Adobe Reader DC.
In mobile devices, such as tablets or cell phones, the users are only able to view and handle a PDF via the Acrobat Reader mobile app.
This app is not feature-rich like the full desktop versionof Adobe Acrobat Pro DC even though they can unlock some basic Adobe Acrobat Pro editing features. They will need a paid subscription to unlock these premium editing features in the mobile app. Otherwise, your users will only be able to view PDFs in their mobile devie(s) via web browser or another third-party PDF viewing app.
To work around this limitation, prepare your forms in Adobe Acrobat Pro DC (full desktop version) but distribute them through Adobe Sign.
You, as the creator of the form, can employ Adobe Sign in your workflow to designate signature and data blocks in a PDF that are only intended for a specific user; this alleviates the issue of having anyone trying to sign on the wrong blocks.
Adobe Sign also facilitates your users with the ability to sign from a desktop computer or from a mobile device using a web browser; it is not required to for them to have a paid subscription of Acrobat Pro DC nor have Adobe Reader DC installed in their system in order to be able to fill and sign the document.
In addition, if you're interested in exploring this featured tool, the business plan allows you to even publish a PDF as a webform, which is very convenient in your case, where you don't know who will be signing on the other end. They just fill and sign online and the document is receipted back to you via email automatically.
If interested, here are some links that I posted before for other users with similar inquiries: