tools
PDF operations, ranked by what they can silently corrupt
Compress, merge, split and OCR each have a failure mode that leaves a file that opens fine and is wrong. Which to fear, and what to check after each.
QR codes: the four decisions that decide whether yours scans
Version, error-correction level, static or dynamic, quiet zone: the QR choices that decide whether a code scans in the field, with numbers from the spec.