xpdf depends on virtual/x11 which will be depricated with the modular X release, it should depend on all the components of the modular X that it depends on
Spyderous has already fixed it: 09:12 < CIA-1> spyderous * gentoo-x86/app-text/xpdf/ (ChangeLog xpdf-3.01-r3.ebuild): 09:12 < CIA-1> Add modular X dependencies. 09:12 < CIA-1> (Portage version: 2.0.53)