[Feature Request] API to Flatten PDF Forms

I haven’t been able to find any way to flatten a PDF form, and I’m integrating with an API (Lob) that can’t / won’t handle non-flattened forms (it won’t render the content of any acroFields). I’ve tried making the fields read-only, and I’ve set NeedAppearances to true but that doesn’t cut it for this one use-case.

Is there a way, even if complicated/hacky to flatten a PDF form with pdf-lib?

[BNovikov]

@patcon , thank you, very much appreciated. I must admit I am still trying to understand the magic of this. But in my case I am trying to put static text instead of the field (would have been very easy if pdf-lib could delete fields), not to have different values in same ids, so that approach would not work. @Hopding, thanks for clarification, I think I’ll try the approach you suggested as well.

[Hopding]

Hello @VivekRajagopal!

Flattening a PDF form just means flattening each field in a PDF’s form. pdf-lib already has APIs to get a list of all fields in a form.

To flatten an individual form field, you need to first ensure that it has an appearance stream. Most software (including pdf-lib) used to fill form fields generate appearance streams. However, pdf-lib does have APIs to generate appearance streams for filled fields that do not already have them.

Once you’ve verified the form field has an appearance stream, you need to take the FormXObject containing the appearance stream and draw it at the end of the page’s content stream. pdf-lib has APIs to get the FormXObject of an appearance stream and APIs to draw a FormXObject on a page’s content stream.

Be sure that you draw the FormXObject in the correct location on the page’s content stream with the correct rotation.

There are a number of libraries written in other languages that you can use as reference implementations of form flattening. I would recommend looking at pdfbox in particular. I used it as a reference when building the form creation/filling APIs for pdf-lib. Here’s a link to the source for their PDAcroForm.flatten() method: https://github.com/apache/pdfbox/blob/trunk/pdfbox/src/main/java/org/apache/pdfbox/pdmodel/interactive/form/PDAcroForm.java#L226-L255

I hope this helps! Please let me know if you have any additional questions.