Hello,
The plugin "PDF serial" is working like the "attach serial" plugin.
When you use the "PDF template", the system loads FPDFi in order to load the first page of the PDF you gave and use it has base.
Afterwards, the plugin starts at the position 0x0, like the other plugin.
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.