Welcome to the forum.
Short of having a third party conversion software, iges or step for part/assembly files
and dxf for drawings files.
The best bet is to sending them a STEP or a Parasolid.
Keep in mind they won't receive feature history with a neutral format.
If you send it as a Parasolid/STEP file. Convert the file and use FeatureWorks to change to SolidWorks features.
Just a thought, I have not try this process.
FeatureWorks has been improving thru the years but it is best suited for basic geometry, not complex models.
Parasolid is the best export/import option to use, as it is the native SW kernel format.