okToAddNotes

Check to see if the PDF file allows adding notes.
[read-only property] okToAddNotes([out, retval] VARIANT_BOOL *ok)
This function returns false if the PDF file is encrypted and does not allow adding notes. The owner password can be used to circumvent this: if a valid owner password was supplied to loadFileWithPassword or loadStreamWithPassword, this function will always return true.
VB6:
If Not pdf.okToAddNotes Then ' can't add notes to this document ... End If