I am a big fan of the FusionInvoice project. I have been using InvoicePlane for a while but moved to FusionInvoice mainly for the multi-currency support and for the continued updates. Support is also extremely reactive and helpful, which for me is super important. The expenses feature is also proving to be very useful, so I am overall very satisfied with it.
Installation is really smooth and the documentation is on point, with the exception of a little hiccup right after you go through it. After installing FusionInvoice and creating an invoice, clicking on the PDF button to export the invoice as PDF would result in the web server throwing an error.
By default, debug logging is not enabled in FusionInvoice. After enabling it, I could see that the issue had something to do with the PDF library in use. Also by default, FusionInvoice uses dompdf, which requires an additional package to work correctly.
I am running FusionInvoice on Ubuntu Server 16.04.2 LTS, so in my case the missing package php7.0-xml, which you can easily install by running
sudo apt-get install php7.0-xml
If you are running another OS, this Stackoverflow question has a few more package names for additional distributions which you can try. Once you install the correct one for your system, you will be able to generate PDFs from invoices without errors.