Best way to create/fill-in printed forms and pdfs?

You may try Docotic.Pdf Library. This library allows to fill in forms in existing documents, import and export FDF data, as well modify existing documents and create forms from scratch.

Several samples:

How to fill in existing forms

How to import FDF to PDF document

How to create text fields

Docotic.Pdf comes with commercial and free licenses.

You can use a recently created Kevsoft.PDFtk package which wraps up PDFtk server.

var pdftk = new PDFtk();

var fieldData = new Dictionary<string, string>()
{
   ["Best Coding Website Box"] = "Stackoverflow",
   ["My Check Box"] = "Yes"
};

var result = await pdftk.FillFormAsync(
   pdfFile: await File.ReadAllBytesAsync("myForm.pdf"),
   fieldData: fieldData,
   flatten: false,
   dropXfa: false
);

if(result.Success)
{
    await File.WriteAllBytes($"{Guid.NewGuid()}.pdf", result.Result);
}

There's more sameples on the GitHub page - https://github.com/kevbite/Kevsoft.PDFtk

Also, there's extra information on this blog post - https://kevsoft.net/2021/05/16/filling-pdf-forms-for-free-with-dotnet-core-and-net-5.html

Kevsoft.PDFtk is licensed under MIT

PDFtk Server is licensed under GPLv2, however, if you're packaging up with an application to be distributed you can buy a redistribution license.

If your form is based on the AcroForm technology: Just use the itext7 to accomplish this task. Add it to your project by executing following command in your NuGet Package Manager Console:

Install-Package itext7

To write a specific form field, use code similar to this:

PdfReader reader = new PdfReader(src);
PdfWriter writer = new PdfWriter(dest);
PdfDocument pdfDoc = new PdfDocument(reader, writer);
var form = PdfAcroForm.GetAcroForm(pdfDoc, true);
var fields = form.GetFormFields();
fields.Get(key).SetValue(value);
form.FlattenFields();
pdfDoc.Close();

In this snippet, src is the source of a PDF file and dest is the path to the resulting PDF. The key corresponds with the name of a field in your template. The value corresponds with the value you want to fill in. If you want the form to keep its interactivity, you need to remove the form.flattenFields(); otherwise all form fields will be removed, resulting in a flat PDF.

Caution

Be aware, that itext7 is licenced under AGPL and isn't free for commercial use or closed source. (special thanks to @da_berni for this necessary information)

There are several options here.

1) FDF, Form Data Format. And that's a terrible spec document, it only covers a small (infrequently used, complicated) part of the FDF format. FDF files are fairly trivial to generate, and contain a pile of field/value pairs (and can hold list options, and other fancier stuff you won't need) and a file association. Opening the FDF fills the PDF (via a file association with acrobat/reader).

Here's a sample (with extra whitespace to make it more readable):

%FDF-1.2
1 0 obj
<< /FDF
  << /Fields  [
    << /V (Communications Co.)/T (Address1)>>
    << /V (29 Communications Road)/T (Address2)>>
    << /V (Busyville)/T (City)>>
    << /V (USA)/T (Country)>>
    << /V (24 January 2000)/T (Date)>>
    << /V (Suzie Smith)/T (Name)>>
    << /V (\(807\) 221-9999)/T (PhoneNumber)>>
    << /V (777-11-8888)/T (SSN)>>
    << /V (NJ)/T (State)>>
  ]
  /F (TestForm.pdf)
  >>
>>
endobj
trailer
<<
  /Root 1 0 R
>>
%%EOF

"/V" indicates a field value, "/T" is a field's title. "/F" is the path to the form to be filled.

There are a number of mail-merge-esque products floating around that can take in an FDF and PDF and produce a filled PDF form. The aforementioned iText (and several others) can do this programmatically, other apps have command lines.

Any page that might need to be repeated should be it's own form in this environment. Merging forms can be Quite Hard. There are a couple approaches, the easiest of them being to "flatten" the fields so they are just page contents (line art & text)... then you're not really merging PDF forms any more.

Of course if you can control the order in which things are printed, you needn't merge the forms at all. You could just open/print them in the correct order.

As I recall, Acrobat Pro's batch commands can import FDF data and print. All you'd need to do would be to generate the appropriate FDF files, which is mostly trivial string building.

Using FDF presumes you have the PDF forms already made, just waiting to be filled out. If that's not the case...

2) Generate your PDF forms programmatically. I do this with iText (the Java basis of iTextSharp), though there are quite a few libraries available in various languages. iText[Sharp] is licensed under the AGPL (or commercially). With AGPL, anyone with access to your OUTPUT must have access to the source of your application. AGPL is just as "viral" as the regular GPL. Older versions were available under the MPL.

Given that this is strictly internal and that you'll be printing the PDFs, the licensing isn't much of an issue.

It would be considerably more efficient to generate your form templates once then fill them in... either directly or via FDF.