For you to make an updated version of it, or them?
I mean, compression of the file would be fine? lots of settings to reduce file size of pak files - but if you want someone else to edit the file? They will need the umap file to do it.
You personally will use your own umap files and these will be a better version of the *.t3d file.
In summary:
*.umap = editable map file with references to project file
*.pak = compressed file with everything needed to run, but compressed…because it has been ‘packed’.
You cannot edit a *.pak file and you cannot play a *.umap file