There is a chance that the PDF is too large to email. You can instead use Apple's File Sharing feature through iTunes. You can do this by following these steps:
1) Connect your device to your computer and open iTunes
2) Select your device then "Apps"
3) Scroll to the bottom of this page to "File Sharing"
4) Open our app on your device then select our app in the menu under "Apps"
5) Select the PDF and then tap the "Save to..." button and you can then save the PDF