Hi,
What you write in the "Your shop name & address" setting of your HikaShop configuration will appear at the top left of your invoices generated by HikaShop.
On our website, we include the VAT number of our company there. And you can add more details if necessary.
A PDF invoice editor would require months of work of a full time skilled developer. An normal HTML editor can't be used as is, because it contains PHP code and custom HTML tags (required by the HTML2PDF library used by the plugin). I don't see this happening. It's too much work for little gains. In fact, in order to cater to people like you, we've added several layouts to the PDF invoice plugin with settings to change the colors. We've also added a "Your logo URL" setting to the HikaShop configuration, which is taken into account by the PDF invoice plugin. With all this, you can already personalize the PDF invoice quite a lot.
Anything more will require modifying the code of the invoice template file of the plugin I'm afraid.