Hello,
It indicates that something is generating data before the plugin ; like a third party plugin which would "echo" some content.
Because of that, the lib cannot send the headers so it can't send a PDF file instead of an HTML document.
php.net/manual/en/function.header.php
You should check your template and third party plugins ; you might need to perform a bit of debug in order to know which file did send some data to the client.
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.