Find and download lots of sample files

To test our software with some real-world data, I want to download lots and lots of files of a specific type (say, PDF or doc).

I could manually google and use some download manager extension for my browser, and have in fact done just that, but it doesn't really scale.

Is there a way to automate this process? I do not care about the content at all, I just want to find and download as tens of thousands of realistic and diverse test files.

I'm on a Mac, but we also have Windows machines available.