Paypal has a credit line known as Paypal Credit which allows you to make purchases through Paypal and then pay monthly payments on the line of credit that Paypal loans to you.
So if you sign up for and qualify for Paypal Credit and use the Paypal Credit responsibly and pay it back like you're supposed too and on time then yes you can build your credit up some using Paypal Credit.
When you first sign up for Paypal a hard inquiry will be placed on your credit report that will lower your credit score by just a little bit but after awhile it should no longer affect your credit score and you should see your credit score increase.