The main issue on our end is that such PDF library weights several MB. DomPDF for example weights 7MB when compressed. That's more than 3 times the weight of the whole HikaShop install package. Including such PDF library would create lot of install issues on servers with heavy restrictions. So we can't include that by default in HikaShop. It would need to be done as a plugin. Also, usually, the rendering from HTML to PDF is less than perfect and thus requires a separate view from the default HTML view. So that's again more complex for the users who would have to apply their modifications to both files (and the same for new features on our end).
Nothing is impossible of course, it's not as easy as it sounds, as you have seen.
Unfortunately, you see is as a very much demanded feature, but it's not. We only had handful of requests for that and we try to focus on what has been requested more first.
If that's something that you really would like to see rapidly for HikaShop, I can only invite you to seek help with third party developers like polishedgeek.com
They would be able to develop it as a plugin for you.