Which is the correct way of saying,
the document can be found in www.example.com/ark
the document can be found on www.example.com/ark
the document is located at www.example.com/ark
Since URLs give the location of an internet resource,
"the document is located at www.example.com/ark"
is correct. If instead of the URL you were using the name of the website, eg. 'Ark', then you could say:
"the document is located on Ark"
I would say that "at" is your best choice because it is "at" a particular location.