Yes, Parimatch is as legal as other international betting sites. The only 2 parameters that legal betting sites must uphold are: Must be based abroad (outside India) and must accept Indian Rupee deposits.
Go to Settings and open the Security section. Scroll down to find the Unknown Sources option. Tap OK/Allow. Go back to PariMatch apk which is stored in the Downloads folder. Run the installation and wait for it to complete successfully.