My understanding is that the LFD file contains all of the information from the design in Author (all the content/formatting/timing etc). By building a single file like this, it should allow the Monitor to read everything in an already compiled format, otherwise you would have to transmit each individual file (the Photo's, videos, documents etc) and the Monitor would need the brains to be able to reassemble everything (think of having a video embedded in a powerpoint then moving the file location of the video, the video will no longer play. However, if you save your powerpoint as a "show file" it includes a copy of the already embedded video, so the video will work even if you moved the original). As far as transmission goes, I'm not sure if there is compression, but if you are multi-point addressing, it should be capable of transmitting the single file and having the multiple units pick it up. I wouldn't be surprised if there was packet numbering happening, given the nature of the product being for signage, so any transmission should be including on the Tx "Sending packets 1-50 of 300" for example, and only continuing on once it gets a received response from all units on the Rx feed.
I'll do some digging and see if I can find anything further on this, hopefully some of the above makes sense though.