Hi,
Could you please configure your web site to use extension-less URLs (
http://devnet.kentico.com/docs/devguide/configuration_of_extensionless_urls.htm) and then try to change the .aspx extension of inserted link to .pdf ?
Could you please also try to store your document attachments on file system and use "Redirect files to disk" option? This could also help you.
However, there were some issues in Kentico CMS regarding pdf files in previous versions of Kentico
CMS, so I would recommend you to upgrade to Kentico CMS 5.5 where I’m not able to reproduce this issue even if the link to pdf file contains aspx extension.
Best regards,
Miroslav Remias