CutePDF Form SDK
The easiest and most affordable PDF forms technology available.
From: cutepdf.com

Author opinion:
"This SDK is the most easy to use Form SDK i have tested, it is very powerful because it is easy to learn and apply!, with a few lines of code you can build a complex forms application.
If you have a paper forms and want to convert it to fillable form, just scan and import it to the FormMax tools and place your fields by WYSIWYG editor, name the fields and publish it to PDF, HTML,... !!"

Company description:
CutePDF Form SDK allows you to empower your applications with the best PDF forms technology. Whether you need to populate a PDF file's form with data, stamp images, retrieve data from an interactive PDF form, flatten a completed form, set preferences or add security to your PDF documents – with CutePDF Form SDK, you can handle it all with just a few simple API calls.
 
CutePDF Form SDK is based on the native COM (ActiveX) architecture and provides dual IDispatch interface, callable from Visual C++, Visual Basic, Visual FoxPro, VBScript, Active Server Pages, .NET, Delphi, etc. It's perfect for multi-task, multi-threaded server applications.

Features Highlight

• Populate a PDF file's form fields by familiar name-value pairs and create a new completed PDF form or file. You can set both value and formatted display string of a field.
• Stamp images into the specified form field.
• Set the general options of form objects and modify the properties of a form field.
• Set the default value of fields. 
• Return the type of form object by name. 
• Set and/or get the list of items of combo boxes or list boxes.
• Import form data from a FDF, XFDF or XML file into a PDF template and create a new merged PDF form or file;  Import form data from a memory buffer.
• Extract form data field by field directly from a PDF form or from a FDF/XFDF/XML file.
• Export data from a PDF form and create a new form data file in FDF, XFDF (XML-based FDF) or XML format; Write form data to a memory buffer.
• Create a FDF, XFDF or XML data file from name-value pairs.
• Flatten a PDF form to remove the fields and stamp the content into the document.
• Add security (native 40-bit & 128-bit PDF encryption) to your PDF forms to prevent anyone from changing it or altering the information.
• Retrieve or set the information of PDF documents.
• Write a completed PDF file to a memory buffer.
• Digitally Sign a PDF Document.
• Security Features: Add security (40-bit & 128-bit) to any PDF files; Change or Remove the encryption of your PDF documents.
• Utility Features: Merge PDF documents; Split PDF document.
• Utility Features: Merge interactive PDF form file.
• Set the opening display options (Viewer Preferences) for the document.
• Stamp stylized text directly on the specified page.