As per the issue description mentioned above, you are unable to send a pdf file as an attachment using send email option, is that correct?
Could you let us know the application you are using- Adobe Reader or Adobe Acrobat?
Try the troubleshooting steps given in the following help document:
Also, let us know dot version of Acrobat or Reader and the operating system installed on the machine? You may refer to the steps given in this link on how to check the version: Identify the product and its version for Acrobat and Reader DC
You may also visit the following forum threads discussing the similar issue:
Let us know if you need any help.