Hello,
It is not possible to create a multiple-page PDF with PDF Serial.
The way than PDF Serial is working (using the same base than AttachSerial), and the trrigers in HikaSerial (one call per serial) ; define the plugin behavior.
It would require an important development to let the plugin have two different kind of workflow ; and to keep the retro-compatibility if the feature is enabled but a customer try to perform a download from an old order.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.