Adobe is planning to patch a fairly low severity security vulnerability in all of the current versions of Reader and Acrobat that could enable an attacker to track which users have opened a certain PDF document. The vulnerability can t be used for code execution, but researchers say it could be used as part of a larger attack. Although the issue is not a serious problem (such as allowing code execution), it does let people track the usage of a PDF. Adobe on Thursday acknowledged the issue and said that it will patch the vulnerability in its next scheduled Reader update on May 14.
Source: https://threatpost.com/adobe-to-patch-reader-information-leak-bug/100143/