I am having this same issue as well.
I create a PdfDocument and then do the following:
Code:
// Access the AcroForm
var form = document.AcroForm;
form.Fields["StudentName"].Value = new PdfString(student.FullName!);
I have tried some of the suggestions mentioned on this forum and elsewhere but with no luck.
I've tried doing document.Flatten(), however this only flattens the fields for the file if it is opened up in a browser not in Adobe Acrobat.
The values are there on the pdf when opened in Adobe Acrobat, however I am unable to view them unless I click on the specific field and I believe this is due to that field not being flattened.