Since the URL within the PDF file is trying to harvest credentials, it can be considered as intelligence gathering rather than dropping a malicious content. This is a phishing strategy used by possible attackers.
Unfortunately, we cannot sandbox a URL that displays a login page to enter credentials. This is a not a sandboxing defense. The DDAN AI cannot enter an email address and password to input into a login page. Smart Protection Network won't be able to detect this URL. It is not possible to source every potential phishing URL.
The Web Reputation is not a filter. It is a database in the cloud which is queried by lookup and scores given for URLs. Thus, the undetected phishing link within an attached PDF file in an email is a normal.
For such phishing link, file a threat case to Trend Micro Technical Support. The URL should be uploaded and classified as phishing. The Threat Team can further check the URL and if a file is downloaded, a pattern-based detection will be created for Trend Micro products.
For more information, refer to the Deep Discovery Analyzer 6.5 Online Help page.