Alfred there is a facility  within  PagePlus that lets you create and use electronic-based PDF forms to collect inputted data from readers of your publication in an efficient and modern manner. In much the same way as traditional paper forms are used to collect information.

Didn’t that use Serif Web Resources (SWR), a service that no longer exists? There’s almost certainly another way, but it would probably involve setting up some kind of data-handling provision on your own web hosting space.

Perhaps you should look into alternatives such as Cognito Forms.

Within the PDF file
Alternatively, it is possible to store form data within the PDF Form itself by using the Save or Save As command. One condition of this is that the form recipient must be using Adobe® Acrobat® Pro (6.0 or later).

Much easier to use a PHP script to save data or send email responses.

That's not something I've ever done, but at the risk of stating the obvious, have you looked up PDF Forms in Help? There is quite a bit there.