One of the most popular best free PDF reader apps on the Google Play Store is actually just a vehicle to deliver potentially harmful adware, experts have warned.
An investigation by Malwarebytes found that the imaginatively-named "PDF reader - documents viewer" app did nothing of the sort, but instead flooded user devices with annoying full screen adverts, even when not in use.
The company warns that the app has already had over a million downloads, and Google has yet to remove it, meaning even more victims could be hooked soon.
Pain-DF reader
In a blog post outlining the scam, Malwarebytes notes how the app looks to trick users by not displaying ads immediately after being installed.
Instead, it waits a few hours, making it harder to spot which app has caused the issue. When activated, however, the app fills the user's home screen with full-page adverts, blocking them from accessing their device unless they click on the ad.
Video ads are also displayed, effectively preventing the user from accessing their device, with ads launching more and more frequently the longer the app is left unchecked.
Malwarebytes notes that the initial Google Play app listing page raised a few red flags that may have tipped users off that this was not a legitimate service. This includes a "Mature 17+" content rating - pretty unusual for almost any of the best PDF readers for Android that we've tested, and the developer name of "Fairy games" - again, not something you would typically associate with such a service.
"If you have a lot of apps installed on your mobile device, this one can very hard to track down," Malwarebytes noted. "Another reason to not blindly trust you are safe while installing exclusively from Google Play. Even if the Play Store is by far the safest place to install apps on Android, it can fault from time to time as well. Having an anti-malware scanner, or anti-adware in this case, is a good idea. Stay safe out there!"
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