From
this link:
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"There are exceptions. Preview can't open protected PDF files, and it may not render all PDF documents properly. The PDF file format has become extremely complex over the years, including support for JavaScript, embedded Flash content, and other advanced options. Acrobat includes far more extensive controls and content capabilities than simply printing to PDF, especially if you need to manage image resolutions and formats. Of course, this complexity in Reader and Acrobat is where a lot of these security problems come from in the first place.
Since there is no risk unless you open a malicious file with Reader or Acrobat, one of the best steps you can take to limit the chances of future issues (aside from staying up to date with patches) is to set Preview as your default reader. Not that Preview is perfect, but we have yet to see it face the same number of zero day vulnerabilities or exploits."
