PDF files edited by Apple Pencil too large to email

I recently purchased the iPad Pro and Apple Pencil for my children to complete school work at home. The work is set by the school as a pdf and when the Apple Pencil is used to mark up the papers provided the size of the file increases from say 430kb to 90Mb and are too large to upload or email. How do I edit pdf documents using the Apple Pencil but keep the file within a reasonable size to email back to the school?

Many Thanks

Malcolm