tl:dr: try a different font type
I have this same issue running KDE Plasma.
As @guiverc insightfully mentioned:
It is documented in the Lubuntu 20.04 release notes, see
When I visited the known issues link, more details are provided. To quote directly from that page:
LibreOffice Exporting Documents as a PDF
There is a bug where certain fonts do not render in exported PDF documents. As a workaround, LibreOffice applications can be launched from the commandline with the following environment variable SAL_VCL_QT5_USE_CAIRO=true. For example, to launch writer issue SAL_VCL_QT5_USE_CAIRO=true libreoffice –writer.
To emphasize in the first sentence there, it explains that the issue is with "certain fonts". In my case, I was using "Cantarell". I changed the font type to "Hack" and presto! My Libreoffice Writer and my HP printer started printing properly. The pages are no longer blank and the content is printing as expected.