You can customize the default naming scheme of the generated PDF documents for different type of Outlook items such as mails, appointments or tasks separately.
Under Email to PDF toolbar/ribbon > ‘Output Settings’ > ‘File Default Naming’ section, there are two drop downs from which you can choose two metadata fields, and these two portions would be concatenated by the chosen symbol of the separator drop down. The availability of the fields would depend on the Outlook item type.
For example, for appointment, you will have the entire fields specific to that Outlook type, such as Start time, End time, Organizer etc.
For more information, refer to the product page at https://www.assistmyteam.com/email-to-pdf-for-outlook/