How to flatten a PDF form in C#

The shortest working example

If the PDF contains an AcroForm, flatten it before the final save.

using ZingPDF;

using var pdf = Pdf.Load(File.OpenRead("form.pdf"));

var form = await pdf.GetFormAsync();
if (form is null)
{
    return;
}

await form.FlattenAsync();
await pdf.SaveAsync(File.Create("form-flattened.pdf"));

Flatten when the output should stop behaving like a form

Flattening makes sense when the completed file is going to another system, to a customer, or into an archive and nobody needs to edit the fields any further.

If the next step still needs field values to remain editable, keep the form interactive and save without flattening.

Flatten after you set the field values

The usual sequence is fill first, flatten second, save last.

using ZingPDF;
using ZingPDF.Elements.Forms.FieldTypes.Text;

using var pdf = Pdf.Load(File.OpenRead("form.pdf"));

var form = await pdf.GetFormAsync();
if (form is null)
{
    return;
}

var nameField = await form.GetFieldAsync("Applicant.Name");
if (nameField is not null)
{
    await nameField.SetValueAsync("Taylor Smith");
}

await form.FlattenAsync();
await pdf.SaveAsync(File.Create("form-completed.pdf"));

What flattening changes

ZingPDF writes the current widget appearances onto the page as normal content, removes widget annotations, and removes the form structure from the saved document.

That means the result looks like the completed form, but no longer exposes those fields as editable AcroForm controls.