Something strange is on my website. How do I get rid of it?

A friend told me, google my site with the following words added:

edberry.com merisa wolf

Select the 4th entry, which is:

[DOC]

DISCLAMER: the following is for educational purposes … – Ed Berry

edberry.com/…/Intructions-for-Acceptance-of-Warranty-Deed…

And then download a Word file that I did not put on my site. (I am not in the warranty deed business.)

edberry.com is a multisite.

How do I find where this document is on my site?

How do I find and remove other such unwanted documents?

How do I prevent hackers from adding material to my site?

Thanks,

Ed