The PDF is not editable in Laserfiche regardless. You can only add annotations within LF, so editing the actual PDF would require export/reimport. You have a couple options to do this where option 1 or 2 is preferred:

1. Use a metadata field to capture the notes, so the users open the PDF and open the metadata pane on the right to read/add notes
   1. caveat is all notes would be editable and anyone could remove/modify existing notes
   2. Multiple users editing notes at the same time would lock the entry, only one person can edit the metadata at a time
2. Use a metadata field and an embedded form. The form would populate a collection with the previous notes, and allow for one or more additional notes. On submission update the entry's metadata
   1. More complex setup for the form
   2. Not necessarily real time, could workaround realtime updates to the collection but adds complexity
3. Build out the PDF in a form and use workflow to overwrite the original PDF
   1. Complex to pre-fill the form with PDF data, doable but definitely the most complex of the options
   2. Users cannot view the form directly in the repository so they would see the PDF and then need to edit it in a form
      1. This probably makes option 1 or 2 more accessible if this is what they have to do anyway

If the PDF is a part of a larger process and has a lifecycle, i.e., can only have notes added during some active time frame, you could use option 3 where form fields create/recreate the PDF on any submission