Here are a few failed and successful ways to specify a linke to a PDF file in your repo. The failed ones will all lead you to 404.
Failed link: maths/en_nn_andrew_ng.pdf
Failed link: ./en_nn_andrew_ng.pdf
Successful link: /assets/maths/en_nn_andrew_ng.pdf
Successful link: /maths/en_nn_andrew_ng.pdf