I would like to be able to send design candidates more information (files & pictures) concerning the scope of the project for their better understanding of what is required. How do I do that?
Once you accept freelancer's proposal or they accept your invitation to apply for your job, an interview room will be created and you can use it to exchange messages and files with the freelancer.
Speaking as a freelancing developer, it's always very useful to see design documents attached to a job description. It indicates that the client is serious about the job, and thoughtful to boot. It can make for much better informed bids.
IMO it usually makes sense to make those documents available to all. However, if you just want to provide them to a particular freelancer who has made a proposal, you can attach them to a message to that freelancer.
re: "I would like to provide a much larger number of documents than Upwork appears to allow, to all potential developers. Any way that I can do that?"
Put them in directory using FTP. Provide a link to the directory that potential developers can view using a web browser.
Or upload a GitHub repository.