Access is strictly limited to National Civil Aviation Authorities and authorized entities (such as airports and aircraft operators).
The document covers topics such as:
: Provides guidance on implementing the security-related Standards and Recommended Practices (SARPs) found in Annex 6 — Operation of Aircraft . icao doc 9811 pdf link
: Detailed protocols for maintaining the integrity of the flight crew compartment. Access is strictly limited to National Civil Aviation
ICAO Doc 9811, titled the Manual on the Implementation of Security Provisions of Annex 6 icao doc 9811 pdf link
Alternatively, sometimes public libraries or educational institutions have access. If the user is affiliated with such institutions, they might be able to request it through interlibrary loan. Or there might be non-official repositories where the document is shared, but I should avoid those due to copyright issues.