For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ChristianMauderer:opensuse-installations/openSUSE_Tumbleweed/home:ChristianMauderer:opensuse-installations.repo
zypper refresh
zypper install diffpdf
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ChristianMauderer:opensuse-installations/15.5/home:ChristianMauderer:opensuse-installations.repo
zypper refresh
zypper install diffpdf
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ChristianMauderer:opensuse-installations/15.4/home:ChristianMauderer:opensuse-installations.repo
zypper refresh
zypper install diffpdf