[SOLVED] Does Ghostscript support inserting JavaScript into an existing PDF?

Does Ghostscript support inserting JavaScript into an existing PDF?

I do not find anything related in the documentation: https://www.ghostscript.com/doc/current/Use.htm

I did find a somewhat related example for inserting watermarks on each page, so it seems like it may be possible.

Is it possible in Ghostscript to add watermark to every page in PDF

However, it's not clear from that example how to insert Javascript.

Solution

If you want to execute JavaScript on document load you can do it like this (thanks to this useful PDF primer).

[ /_objdef {MyAlert} /type /dict /OBJ pdfmark
[ {MyAlert} << /JavaScript << /Names [ (MyAlertCode) << /S /JavaScript
/JS (app.alert\("Hello world 1."\);app.alert\("Hello world 2."\);) >> ]
>> >>
/PUT pdfmark
[ {Catalog} << /Names {MyAlert} >> /PUT pdfmark

Full steps to reproduce would be:

Make sure Ghostscript is installed on your computer (the gs command)
Put your source PDF in a folder alongside a postscript (.ps) file that contains the code from above.
cd into that folder
Run: gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=newpdf.pdf myps.ps mypdf.pdf (obviously replace myps.ps and mypdf.pdf with your actual file names).
Open newpdf.pdf to verify that it worked!

Other tips:

Acrobat doesn't use exactly the same syntax as you might be used to in traditional web development (hence app.alert in the example rather than the traditional alert().) Don't just assume the function you're trying to use exists.
You'll need to use backslashes to escape parenthesis in your JavaScript (e.g. app.alert\("Hello World"\)
You can set up a Javascript console on Acrobat to help with JS debugging

Resources: