Browsers have their way in rendering headers. There seems to be a limit of 250px as tested by @BawarYassin:
The internal PDF generator used to handle cases where the headers are more than 250px but it too had its pros and cons and now it’s being phased out.
You could write a theme to solve this issue, but themes too are being phased out so it’s better to make do with the internal PDF generator until a replacement for themes is announced.