Ensure that the word "stable" is part of the URL; if it isn't, click on the article title or the "Item Information" link. Copy the URL provided, then it paste into the body of your email.
The document creator may have set the security to disable this function. To check, open the file in Acrobat, select File/Document Properties and select the Security tab. Look to see whether Content Copying or Extraction is set to Allowed.