将大字符串存储为PDF元数据？

I need a method for embedding and extracting large strings of arbitrary data within a PDF document. Is it possible to repurpose an existing metadata field, and if so, which one is best? Or is there another way to store/extract non-displayed data within the file?

Some details:

• I'm currently using DOMPDF to generate the PDF documents (from form data).
• The data strings are serialized->encrypted->base64.encoded form data.

Here's a plain text example of the sort of string I need to embed/extract from a generated PDF document.

I'm aware of this post and the $dompdf->add_info() method. What I'm asking is whether or not I can use a DOMPDF-supported metadata field to store and extract a large string, or if there is an alternative strategy altogether.