If you download the Update from 3rd party unverified websites, then yes, you will end up having viruses on your PC. Always use the developer's website to download the update, or use the built-in Updater. Whenever a new update is pushed to public, the Adobe Updater will inform you in the icon placed in System Tray.
Additionally, you can update the software by downloading the official package from the website: https://get.adobe.com/reader/
Note: Make sure to disable the optional offers.