Short answer: Yes, you can, but your client must also agree to the terms of our Licence Agreement.
Detailed answer: We are square if our graphics remain integral to the design. What it practically means:
— One cannot extract files
— One cannot use files again
— One cannot redistribute and resell files (requires written consent from Icons8)
If your clients want to extract and reuse graphics, they must keep an active subscription throughout the entire usage period of stand-alone files.