I used to use a file compression program a *long* time ago that would compress multiple files in different orders and this would give rise to a small gain in compression at the expense of the time spent performing the extra calculations. I can't remember the name of the program.
It worked because the compression algorithm in use in most popular compression programs would produce different results for different (but similar) inputs. For example, feeding in file 1, then file 2, then file 3 might produce a compression of 55%, whereas feeding in file 2, then file 1, then file 3 might produce a compression of 60%. In order to work out which set produced the best results you had to actually compress them . . . so it took a long time.
There could be many different solutions to your problem as outlined above, but if you need to send native autocad drawings (and not .PDF, .JPG or .TIFF version of the resulting image) then you are going to be stuck with a large source file. You might squeeze a few extra %s in compression out by using different tools but most of them use similar algorithms so the difference will be marginal. If you just need to send someone the picture (as distinct from the source file - in other words they don't need to edit the file, just look at it or print it) then you would probably save a lot of space by saving in another format such as .JPG.
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