The first you must use the function "core_course_get_content".

This function will return a lot of fileutl from a course.

Ex: 1 file URL

http://<your moodle>/webservice/pluginfile.php/37/mod_resource/content/1/BTXDPMHDT.pdf?forcedownload=1

and then, You just add a token into URL

http://<your moodle>/webservice/pluginfile.php/37/mod_resource/content/1/BTXDPMHDT.pdf?forcedownload=1&token=6a18222af87608fe4d2ab2ca7ab1f49a