I'm afraid... this trick, worked only for a couple of hours. I can't explain why xpdf was working well...
--- the following text, will NOT SOLVE the problem ---
I've just found the final solution, at least for me.
This morning I've been working with many pdf files, and I was getting tired of xpdf problems...
So, after 15 minutes of damned work I decided my self to solve this issue.
From an ftp mirror site of Slackware, download (wget) the whole xpdf source directory
wget --mirror -nH -np --cut-dirs=7 ftp://ftp.heanet.ie/mirrors/ftp.slackware.com/pub/slackware/slackware_source/xap/xpdf/
Now, use the "xpdf.Slackbuild" to compile and create the package just like the official way.
Remove the installed xpdf and install the file "/tmp/xpdf-3.02pl6-i486-3.txz"
...xpdf will no longer eat and stress the cpu.
slackware 13.37 (32 bit) out-of-the-box
notebook acer travelmate 800lci