Increase character limit for bookmarks in InDesign

This is an InDesign user interface limitation. There must be a hardcoded limit to how much can be entered into that abysmally itty bitty small entry box – the programmers decided that 100 characters was enough. You cannot cheat and create a bookmark from selected longer text either, only 100 characters get copied.

However, when writing the bookmark text through a script, you can add up to 255 characters:

and InDesign will not save any more.

Adding even more text with Acrobat Pro DC shows that this allows as much as 751 characters (a weird random number):

and of course even that has been an engineers' decision. Perusing the official PDF specification ISO 32000-1:2008, I cannot find any mention of a "minimal" or "reasonable" or "maximum allowed" length. I don't have the right software at hand (I'd probably have to write it) but I bet it's possible to create a perfectly valid PDF with bookmarks of thousands of characters long. How Acrobat DC is going to react to this is something else ...

All said and done: since the OP is (ab)using the bookmarks for a purpose they were not design for, it's hardly fair to complain. For my own (daily, and practical) usage, the current limit as enforced by InDesign is ever so slightly just too small, as there indeed are authors who don't mind writing +100 characters long subheaders; but it's of no consequence for me since I don't have to enter the bookmark text (it gets auto-generated by my import process) and if there is an edit in one, well, I have a script for that.

(Also, it seems OP does not mind having 512 character long filenames, which sounds similarly mind boggling to me -- perhaps it is possible, perhaps not. But that complaint is for their OS supplier.)

This is a PDF limitation. My recommendation, use normal headlines as bookmarks, nobody is reading a document in the Bookmarks panel, it makes no sense to use long bookmarks.