Redacting a PDF robustly
Q How can I remove private material from a PDF document, so others can’t see it? Could I use Preview?
by KEITH PARKS
A This is redaction, and it’s fraught with dangers which have led to the revelation of many secrets. The rather naive approach of superimposing black boxes doesn’t work because others can easily remove them to reveal underlying text. The PDF format is far from ideal in this respect, as it’s relatively simple to extract text from sections of a document which you think have been removed, as they’re retained in the file.