How to search a folder and all of its subfolders for files of a certain type

Question

I am trying to search for all files of a given type in a given folder and copy them to a new folder.

I need to specify a root folder and search through that folder and all of its subfolders for any files that match the given type.

How do I search the root folder's subfolders and their subfolders? It seems like a recursive method would work, but I cannot implement one correctly.

user_1415262 · Answer

Another fast way of doing this is delegating the task to the shell command "find" and splitting the output:

pdf_file_paths = `find #{dir} -name "*.pdf"`.split("
")

Does not work on Windows.