Creating custome meta meta data fields

Hello all. I searched on the forums for an answer to this topic however I didn't find any threads.

I want to create custom meta data fields but have not bee able to work out how to do it. Any know how?

I download a lot of PDFs for research and want to be able to have a field in the properties titled "Origin" or similar when I can post a link to the location of where I found the document.

Anyone able to help?