Their security concerns are actually the problem - they don't like installing software on their computer for fear of creating a security problem. My thought was that sending them a password protected link would be the best choice.
A few of us gave usable solutions, i.e. Dropbox, Google Drive, Onedrive, and so on. Only you
have 100% access to all
of the files you upload. You can create a unique link to one file to send to one person (or multiple people). Them having that link does not give them access to any other files.
No software on their end, at least that they don't already have, is necessary.