OK, after double-checking and upgrading almost any package (lacking enough free hdd space I didn't install kde, kdei and xfce package groups) I found that it must be actually the IceWM window manager...
Using twm, xpdf starts fine. Using IceWM, the console (not the xterm) mentions a mere
Quote:
IceWM: Warning: app trying to map XEmbed window 0x1200006, ignoring
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This seems to be a known bug in (grossly) outdated versions of IceWM that lack full qt5 compatibility.
Of course I never updated the IceWM package I got as a Slackbuild (definitely a pre 14.2 build).
I'll try the latest SBo build. Almost sure it will fix the issue... If not I'll try building from latest sources.
Thank you guys, for pointing out this isn't a Slackware issue!