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I have opened and rendered a PDF with a few form fields and the form fields is displayed on the bitmap. However I wonder how to make these fields editable. I believe it has to do with annotations, but not sure which functions to use.

Quick PDF Library enables you to render PDF files. Details on what's involved in actually rendering a PDF behind the scenes, can be found in this knowledge base article about displaying PDF files.

In order to make form fields visually editable, as they are in PDF readers like Acrobat or Nitro PDF, there are a few steps involved:

1. Retrieve information about form fields in your PDF: which types of form fields, what value they contain, where they are located on the page and any other related properties.
2. Render your PDF and then programmatically re-create the buttons, list boxes, radio buttons, text fields, etc, placing them in the exact same position as the form fields in the PDF.
3. Fill the controls that you have positioned on top of the render PDF with the values from the form fields in the PDF.
4. Make sure that if the user changes any of the values in the controls, that these settings are stored somewhere and can be saved back into the form fields in the actual PDF. That's only required if you want the user to be able to save changes.

All of this can be done using the PDF form field functionality available in Quick PDF Library.