Embed File Within PDF

I already have both the PDF and the external file ready; what I need now is a clean, reliable way to bundle them together so the file lives inside the PDF itself.

Please take my supplied PDF, insert the extra file as an embedded object, and return the finished PDF to me. The document must open normally in any standard reader (Adobe Acrobat, Preview, browser viewers, etc.) while still letting users access or download the embedded file with a single click. No page re-flow, compression artefacts, or security warnings should appear once the file is added.

You may use Adobe Acrobat, PDF-XChange, or any other professional-grade tool you are comfortable with—just let me know which one so I can replicate the workflow later if needed.

Deliverables
• Final PDF with the file fully embedded and functioning
• A brief note outlining the steps or settings you used, in case I need to update the document in future

I can send everything the moment we start; this should be a quick, focused task for anyone accustomed to PDF editing.