Typically, this is caused by incorrect linking.  The link will redirect incorrectly if your filename has spaces in it. 

When uploading the PDF, and naming the file, please do not use spaces in between words. Use dashes(-), or use underscores (_), in stead of spaces. The reason being that the filename is "tacked" onto the end of the URL.  The spaces in it, "break" the full link, which then redirects the user to a Blackbaud page. By adding the file with a filename that contains no spaces, and then re-adding the URL link, you will experience a better link to the PDF.