If you don't want to give them the actual files you can open your top assembly in View and Markup and save as a pcf.
You might need to set some of the create options.
Go to Tools > Options then on the View and Markup tab check the boxes for "Store Precise geometry in file" and "Enable Measure, Markup & sectioning"
They can then open the pcf file in view and markup and you haven't given away any intelligent data.
They can also add markups and send the pcf back to you.
bc.
2.4GHz Core2 Quad, 4GB RAM,
Quadro FX4600.
Where would we be without sat-nav?