I've been looking around the internet for information on Koha ILS restful api but I haven't found anything concrete. There is this link which talks about its HTTP API: http://wiki.koha-community.org/wiki/Koha_/svc/_HTTP_API but there are no examples and I'm quite confused with the MARCXML format required.
What I want to do is use this API to create biblio records into a remote Koha ILS system. If I understand correctly, using these services I can create records (probably using a JSON-to-MARC convert tool) but will I be able to also upload pdf files of each record in BASE64 format? It doesn't look like this is possible using this API although I'm not really sure.
The HTTP API available in Koha is a well-established protocol, called SRU, for searching library catalogs. This is protocol is meant only for searching, not for updating records.
Secondly, even though SRU 2.0 provides option for transmission of records in JSON format, most implementations do not support it yet.
Coming back to your use case, Koha cannot store PDF documents. It is a process automation tool in a library for physical collections, which deals only with metadata records. For storing digital documents you should look for document management solutions, such as DSpace, or smaller and easier Omeka. DSpace provides its own REST API for searching as well as supports SWORD protocol for uploading documents.