The report on ad injection, it says, can deliver ads on any website even if it does not run ads.
Google said PPI networks drove more than three billion download attempts during its year-long investigation, and that these networks installed "tens of millions" of downloads during its investigation.
"Prominent strains include ad injectors that laden a victim's browser with advertisements, browser setting hijackers that sell search traffic, and user trackers that silently monitor a victim's browsing behavior," according to the report.
Google named four PPI companies it considered to be the worst: Amonetize, InstallMonetizer, OpenCandy and Outbrowse.
The study was carried out in partnership with New York University and the International Computer Science Institute.
"The major takeaway from our perspective is that sometimes there is a misaligned incentive in the PPI model, where [distributors] are solely focused on trying to drive installs because they get upwards of a $1.50 for a successful install," said Kurt Thomas, a research scientist at Google.
Source: Ad Age