The question was asked before in the Atlassian Answers.
https://answers.atlassian.com/questions/124674/export-to-pdf-and-include-attachments

There is tool that can embed attachment inside PDF
search for attach_files
https://www.pdflabs.com/docs/pdftk-man-page/
But please review if there is licensing issue with pdftk (which use itext5,
https://en.wikipedia.org/wiki/IText)

It seems the old version of pdftk (based on itext2) may not have the licensing issue.
BTW, the 'old' pdftk becomes pdftk-server now, which seems to be GPL but have licensing terms for commercial use..... https://www.pdflabs.com/docs/pdftk-license/

FYI, tcpdf (php) is another library that can attach files into pdf
https://tcpdf.org/examples/example_041/