Lazy reading
What opening a document does and deliberately does not do, what that costs, and what it measured.
Diagnostics
Why the reader repairs what it can and returns an account of what it did, and how it reasons about damaged streams.
Reader limits
How the reader keeps every valid PDF readable while treating every file as hostile, and why its guards are on by default.
Validation
What validation answers that reading does not, where the line between a warning and an error lies, and how the Arlington model is used.