Each chain has its own limits set in the blockchain's parameters. However the absolute maxima supported by MultiChain 2.0 are as follows:
Block size: 1 GB
Transaction size: 100 MB
On-chain single item size: 64 MB
Off-chain single item size: 1 GB
You can store images and/or PDFs in MultiChain using a stream – just post the data as binary into the stream. You can see an example of this in the MultiChain Web Demo which allows files to be uploaded.