emerge should be able to resolve this block, as it is going to upgrade xpdf to >=3.01-r3 anyway clearing it, and so the future emerge unmerge for "=xpdf-3.01-r3" which is part of the emerge -U operation should made the currently required manual `emerge unmerge xpdf` not to be needed.
# emerge -DNvup ">=xpdf-3.01-r5"
These are the packages that I would merge, in order:
Calculating dependencies ...done!
[blocks B ] <app-text/xpdf-3.01-r4 (is blocking app-text/poppler-0.4.3-r4)
[ebuild N ] app-text/poppler-0.4.3-r4 -cairo +jpeg +zlib 806 kB
[ebuild U ] app-text/xpdf-3.01-r5 [3.01-r3] +X 0 kB
*** This bug has been marked as a duplicate of 79606 ***