Copy to External exFat HD creates hidden files

I use Finder on Mountain Lion to copy a Folder (having several subfolders) to an external exFat HD. Along with the copied files the copy is resulting in several hidden files.

For example, this

lightroom_4_image_management_workshop_toc.pdf

and this

._lightroom_4_image_management_workshop_toc.pdf

How can I prevent those hidden files from being created?

How can I delete those hidden files already copies?

Thanks

Mike