Billions of PDF files are exchanged daily and many people trust them because they think the file is "read-only" and contains just "a bunch of data". In the past, badly crafted PDF files could trigger nasty vulnerabilities in PDF viewers. All of them were affected at least once, especially Acrobat or FoxIt readers. A PDF file can also be pretty "dynamic" and embed JavaScript scripts, auto-open action to trigger the execution of a script (for example PowerShell on Windows, etc), or any other type of embedded data.