I guess it's the classic problem of converting from a format to another. SAP's intention is not to deliver converters, and usually you need to use third-party ones. Only a few conversions are possible in SAP (and usually with very limited features), from ABAP list, TXT, OTF, to PDF.
Note: In SCOT settings, you have an option to save the inbound emails into a database table. So, the feature is there without doing anything, you'll have the mails in HTML or TXT format, instead of PDF.