xpress will work fine as a viewer, it will open large assemblies in read only.
However the user will be able to see everything, all the feature history of all your parts and every component. This may not be what you want - in which case you are better using the hsf/modelpress route.
The user could also perhaps delete parts from the assembly down to the parts limit and then edit the parts save them and send you them back.
Note that as mentioned before currently xpress is out of step with v9 so you can't currently exchange files with xpress unless you are still running v8.2, but I assume v9 express will come shortly.
At Alibre.com there are not so many links to xpress. If you found alibre xpress though download.com or suchlike then you will be linked to alibre.com/express and there are lots of directions how to upgrade to the paid versions!