First you need encode your binary files( your pdf files). The base 64 encoding is designed to make binary data survive transport through transport layers that are not 8-bit clean.
I would suggest you to create a function to return one document and return a json object
with some of the attributes of the file( name, extension, date,...etc.., content) and in the content field of the json object, just put the base64 string that you obtained from applying the base64 enconding function to the pdf file. In this way you can (in the client side) decode
this string and get the original file without any problem.
Base64-encoded data takes about 33% more space than the original data.
Because of this I suggest you to return only one document on each call to the RESTful web service. Becasue a pdf file can be small, but it can also be huge, and combining several documents on the response json object could make your transfer very heavy. But this is up to you.