Very interesting. GNUCash provides invoicing features. Would try it out after it hits repo maybe within a week.

I'm trying this solution. It will work for my case of single entry.
1. Create libreoffice writer template
2. Save as fodt file with some keywords within {} like say {hours}, {date}.
3. Replace those keywords within {} with a script
4. Use libreoffice --convert-to pdf foo.fodt .

For more complex invoicing, dynamic table must be made but it looks doable!

I am using it for about 2 years now, in my freelancer part time occupation. It's a little intricated at first look, but you can find help in the project's mailing list,  documentation or in online videos in an invidious instance. There is even a book for gnucash. 

I think my self as an novice+ GnuCash user and I use only the really basic functionalities, like income/expences, invoices etc, but If somebody needs help, I will try to answer.

Automating invoices needs scripting and this is way off my league though.

I'm thinking about having an abiword or libreoffice (soon out according to throgh) template and giving some function an input of billing items, name, address and exporting pdf.

Is there a better way or some free software specifically made for this?