Ambiguities in the PDF specification means that no two PDF parsers will see a file in the same way. This leads to many opportunities for exploit obfuscation. PDFs are currently the greatest vector for drive-by (malware installing) attacks and targeted attacks on business and government. A/V technology is extraordinarily poor at detecting these. The PDF format itself is so diverse and vague, that an A/V would need to be 100% bug-compatible with the parser in the vulnerable PDF reader. You can also do cool tricks like make a single PDF file that displays completely differently in several different readers.
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