Thanks for considering Aspose.
The evaluation versions provide the complete functionality as provided in licensed version except for the watermark inside the resultant document. I'm not sure why you're getting an exception when converting XLS file into PDF format. Can you please share the source file so that we can test the scenario at our end. We apologize for your inconvenience.
You can also get a Temporary 30 days license. For more information, please visit this link.
Regarding your other query on license, I've asked my fellow worker from sales department to answer this query.
In case of any further query, please feel free to contact.