Enough to show what the knowledge is and that he holds it, not the entire system. Representative protocols with revision histories, a document-control extract listing his authorship and a summary table work better than a full manual.
Show the system through samples and an index
Hypothetical example: a Rothesay manufacturer's quality manual runs to several hundred pages and the company is reluctant to place trade secrets in a government file. The petition does not need all of it. A summary describing the validation system, its scope and its proprietary elements, together with three or four representative protocols that he authored, the document-control register showing his name against the documents and dates, training records showing he trained others, and a comparison with the general regulatory guidance is usually sufficient.
Confidential technical content can be redacted where it is not needed to show the nature of the knowledge, provided the redaction is explained. A letter from the head of quality should reference these records rather than replace them. If any records are in French, certified English translations are required for USCIS.
The goal is to let the officer see the knowledge is real and company-specific without exposing more than necessary.