Need to reduce Invoice PDF copy size which is being sent to e-mail

Earlier we are in Optio (3rd Party tool)to get the invoice copy in PDF format that time size of invoice copy was 22KB only, now global invoice form migration happen from 3rd party to Smart form. But now the size of the invoice PDF copy shows as 393KB. Hence requesting you to please let us know, is there any option or OSS note available to reduce the size of invoice PDF copy as earlier.