Adobe’s Acrobat/Reader line of PDF viewers was recently updated to address a single security issue.
Although there are several variations of Acrobat and Reader, the one of interest to most people is the freeware Acrobat Reader DC (Continuous). That’s the one you probably have installed on your computer. The new version for that variant is 2019.010.20098.
Recent versions of Reader seem to update themselves in the background, courtesy of an update service called ARM that gets installed along with Adobe products. You can check which version you’re running by navigating Reader’s menu to
About Adobe Acrobat Reader DC.