To reduce the file size of a PDF, you can reduce the image quality. To do this, tap on the overflow menu button to navigate to the Settings screen, then tap on Reports and finally tap on PDF Photo Quality. Adjust slider as required - to the left will reduce quality whilst to the right will increase it. Reducing the image quality will reduce the file size of the PDF. If the PDF is still too large to email, try exporting the report to an alternative such as Google Drive or Dropbox.