Reliable PDF Attachment for Lead Magnet Freebies in Email Flows

I want to be able to reliably attach a PDF from my Personal Library to an email flow for my Lead Magnet freebie. The current method of uploading directly seems to be unstable, and the file disappears. Using a link from the Personal Library is a workaround, but I need a stable, built-in feature to ensure my subscribers always receive their freebie.