Keeping it simple, I open the GTA mods site, and I'm bombed with Adobe Acrobat logos and adverts on the mod names and the screenshot for the preview. Thing is that it doesn't happen on anything else at all, just Gta 5 mods.com, which made me suspicious. Don't know if anyone else experienced this or has even heard of this before. Perhaps it's just something a person gets when you download a sketchy mod, don't know.
A screenshot would help to form some context, but I suspect this might be local.
Try another browser to see if it still happens.