Hello,
Leave the "filename" field empty and the filename will be the serial.
The setting field "force download of the image", is in fact "force download of the PDF" for the PDF Serial.
Forcing the download is interacting with the HTTP headers to force the browser downloading the content instead of doing a preview.
When that setting is enable, the second setting "filename" is used if you want to set a specific filename ; the serial is used by default.
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.