When you're using a program such as Microsoft Word (or possibly, such as in your case here, a PDF editing one) and then go to File > Send (or whatever) it attempts to open your default email client (such as Outlook or Thunderbird), with a blank email (to which the file is already attached) ready for you to use. Such a system can't work if either
(a) you use webmail, rather than a dedicated email client ; or
(b) you do use an email client but you've not told Windows that you want it used as your default means of sending emails.
In either case, the solution is simple. Save the relevant file to your hard drive (if you've not already done so) and then go to whatever system you use for sending your emails, whether that be a program (such as Outlook or Thunderbird) or your email provider's website. Create a new email there and then click the 'attach' button to append the file to it.