I found an answer for the community version:  the Odoo app store has a free app called Odoo Professional Report Templates.  It is available for all supported versions.  I chose Odoo 10 Community.  I downloaded it, unzipped it, put the folder in the addons path, logged out of Odoo, rebooted my server and client, started Odoo client in browser, turned on developer mode, the app was listed in the Apps tab, I installed it, went to Settings --> companies, chose my company and saw new elements to customize, namely which new report format to choose for each report.  I chose one of them for each report.  Then I went to settings --> reports --> reports, found the new reports, chose the new one for invoices, chose the Q-Web views (top-right corner), chose the view for the report and edited it to put in my custom fields and labels and it worked.  It is easy, doesn't require a lot of programming skills, and you can customize beyond what the app does.

With the Studio app you can customize the report (pdf). Go to your invoice form then hit the Studio icon in the upper right corner, next to your user name (it's a wrench with a pencil). Then go to the report tab and select the report you want to edit (make a copy first).

Hope that helps.