I'm selling PDF's I've created, what's the best WM, one large diagonally across the page, or multipul's ?
Also, how easy/hard would it be for someone to remove it ?
Depends. If all you add is a watermark then quite easy. If you convert the entire page to an image (although that's not very user-friendly) then it's more difficult. If you encrypt the file using a file-open password, even more so. And if you digitally sign the file it will allow you to show that the file has been edited (because it will invalidate the signature).
As to the best way to show it, there's really no answer to that. It's up to your own personal preferences.
Diagonal text is good as it doesn't interfere (that much) with the content of the file and doesn't confuse the user when reading the text, for example.
Thank you for a very consise reply.
Anyway, 'If you convert the entire page to an image' JPEG etc ?
I have a lot so in a batch/bulk in PS or AA ?
Lastly, is there a tut on this topic?
You can do it directly in Acrobat. Go to File - Save As Other - Image. That will convert all the pages to separate image files. Then go to File - Create - Combine Files into a Single PDF and select those image fields, to create a ("flattened") new version of the file.