New Feature - The `load_pdf` working with bitstream of files.

[davibarreira]

This PR modifies the io.load_pdf function to work with files as bitstreams instead of only allowing passing the filepath. This can be useful when one is dealing with files stored in the cloud (my case).

[davibarreira]

It seems that the repo is not setup to run the tests automatically. So I recommend running them before attempting the merge. I haven't actually ran the whole batch of tests locally.