If the PDF is smallish, you can publish it using the straightforward stream APIs, passing the hexadecimal code for the PDF file in the API command. You should go through the first four steps of the 'Getting Started' tutorial for this:
https://www.multichain.com/getting-started/
If the PDF is much larger, you might want to consider using the binary cache, which allows you to push large pieces of data into MultiChain directly via the file system. Use createbinarycache to create a binary cache item, then copy the PDF file into that cache item's file (in the cache directory of your blockchain folder) using the regular file system, then publish it like this:
publish <stream name> <item key> '{"cache":"<cache identifier>"}
You can then call deletebinarycache to remove the item.