Show a PDF form on a webpage, let the user fill the data, flatten it and save the document as PDF/A

Question

Hi I'm trying to achieve the following:

1. Show a PDF form on a webpage.
2. Let the user fill the data needed.
3. Check if all is filled correctly.
4. Save the document as PDF/A.

I have tried many samples, but none seems to work. The problem is not the showing of the PDF. I have already loaded it in an iframe, object, embed..., but I can't figure out how to extract it once it is filled correctly and a Submit button is clicked.

So, does anyone have any working solution in C#, JavaScript , ...