Single thread backtrace, please run "thread apply all bt".
Does the document contain pages which take time (even lot of it) to render?

It's regular pdf file, and it opens fine with kpdf.
But okular can't render some pages. Seems like it stucks on rendering actually.
Unfortunatelly, I can't provide this file as it contains private info.

(gdb) thread apply all bt

Thread 2 (Thread 0x7f9059d6c710 (LWP 7064)):
#0  0x00007f9059ee31a0 in Splash::composite(SplashBitmap*, int, int, int, int, int, int, bool, bool) () from /usr/lib/libpoppler.so.13
#1  0x00007f9059e04ba0 in SplashOutputDev::paintTransparencyGroup(GfxState*, double*) () from /usr/lib/libpoppler.so.13
#2  0x00007f9059e04b3a in SplashOutputDev::endMaskClip(GfxState*) () from /usr/lib/libpoppler.so.13
#3  0x00007f9059e505e5 in Gfx::doImage(Object*, Stream*, bool) () from /usr/lib/libpoppler.so.13
#4  0x00007f9059e50aea in Gfx::opBeginImage(Object*, int) () from /usr/lib/libpoppler.so.13
#5  0x00007f9059e47bd0 in Gfx::go(bool) () from /usr/lib/libpoppler.so.13
#6  0x00007f9059e48005 in Gfx::display(Object*, bool) () from /usr/lib/libpoppler.so.13
#7  0x00007f9059e91796 in Page::displaySlice(OutputDev*, double, double, int, bool, bool, int, int, int, int, bool, Catalog*, bool (*)(void*), void*, bool (*)(Annot*, void*), void*) () from /usr/lib/libpoppler.so.13
#8  0x00007f905a1b6250 in Poppler::Page::renderToImage(double, double, int, int, int, int, Poppler::Page::Rotation) const () from /usr/lib/libpoppler-qt4.so.3
#9  0x00007f905a436205 in PDFPixmapGeneratorThread::run (this=0x1341c80) at ../../../../okular/generators/poppler/generator_pdf.cpp:1851
#10 0x00007f906b10930e in QThreadPrivate::start (arg=0x1341c80) at thread/qthread_unix.cpp:285
#11 0x00007f906828d8ba in start_thread (arg=<value optimized out>) at pthread_create.c:300
#12 0x00007f906a86603d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#13 0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f906d692720 (LWP 7055)):
#0  0x00007f906a85f8d3 in select () at ../sysdeps/unix/syscall-template.S:82
#1  0x00007f906b224c8a in qt_safe_select(int, struct {...} *, struct {...} *, struct {...} *, const timeval *) (nfds=13, fdread=0x12306c0, fdwrite=0x1230958, fdexcept=0x1230bf0, 
    orig_timeout=0x7fffdca7af00) at kernel/qcore_unix.cpp:92
#2  0x00007f906b229e1d in QEventDispatcherUNIXPrivate::doSelect (this=0x1230500, flags=<value optimized out>, timeout=<value optimized out>)
    at kernel/qeventdispatcher_unix.cpp:219
#3  0x00007f906b22a9cb in QEventDispatcherUNIX::processEvents (this=0x1216e60, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventdispatcher_unix.cpp:918
#4  0x00007f906bcc637d in QEventDispatcherX11::processEvents (this=<value optimized out>, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventdispatcher_x11.cpp:152
#5  0x00007f906b1fa222 in QEventLoop::processEvents (this=<value optimized out>, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:149
#6  0x00007f906b1fa60c in QEventLoop::exec (this=0x7fffdca7b210, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:201
#7  0x00007f905eb8cf31 in Okular::Document::closeDocument (this=0x139c430) at ../../okular/core/document.cpp:1761
#8  0x00007f905ee3641d in Okular::Part::closeUrl (this=0x12baef0) at ../../okular/part.cpp:1136
#9  0x00007f905ee36888 in Okular::Part::~Part (this=0x12baef0, __in_chrg=<value optimized out>, __vtt_parm=<value optimized out>) at ../../okular/part.cpp:651
#10 0x00007f905ee368d9 in Okular::Part::~Part (this=0x12baef0, __in_chrg=<value optimized out>, __vtt_parm=<value optimized out>) at ../../okular/part.cpp:676
#11 0x000000000040a03c in Shell::~Shell (this=0x128bfa0, __in_chrg=<value optimized out>, __vtt_parm=<value optimized out>) at ../../../okular/shell/shell.cpp:122
#12 0x000000000040a0e9 in Shell::~Shell (this=0x128bfa0, __in_chrg=<value optimized out>, __vtt_parm=<value optimized out>) at ../../../okular/shell/shell.cpp:125
#13 0x00007f906b20cb98 in QObject::event (this=0x128bfa0, e=0x12306c0) at kernel/qobject.cpp:1194
#14 0x00007f906bc67bef in QWidget::event (this=0x128bfa0, event=0x13d0810) at kernel/qwidget.cpp:8659
#15 0x00007f906c04cdbb in QMainWindow::event (this=0x128bfa0, event=0x13d0810) at widgets/qmainwindow.cpp:1478
#16 0x00007f906ca82883 in KXmlGuiWindow::event (this=0x128bfa0, ev=0x13d0810) at ../../kdeui/xmlgui/kxmlguiwindow.cpp:126
#17 0x00007f906bc124ac in QApplicationPrivate::notify_helper (this=0x12302a0, receiver=0x128bfa0, e=0x13d0810) at kernel/qapplication.cpp:4445
#18 0x00007f906bc18e4d in QApplication::notify (this=0x7fffdca7c270, receiver=0x128bfa0, e=0x13d0810) at kernel/qapplication.cpp:4324
#19 0x00007f906c984436 in KApplication::notify (this=0x7fffdca7c270, receiver=0x128bfa0, event=0x13d0810) at ../../kdeui/kernel/kapplication.cpp:311
#20 0x00007f906b1fb4fc in QCoreApplication::notifyInternal (this=0x7fffdca7c270, receiver=0x128bfa0, event=0x13d0810) at kernel/qcoreapplication.cpp:732
#21 0x00007f906b1fe49b in sendEvent (receiver=0x0, event_type=<value optimized out>, data=0x1217350) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:215
#22 QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=<value optimized out>, data=0x1217350) at kernel/qcoreapplication.cpp:1373
---Type <return> to continue, or q <return> to quit---
#23 0x00007f906b22a960 in QEventDispatcherUNIX::processEvents (this=0x1216e60, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventdispatcher_unix.cpp:890
#24 0x00007f906bcc637d in QEventDispatcherX11::processEvents (this=<value optimized out>, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventdispatcher_x11.cpp:152
#25 0x00007f906b1fa222 in QEventLoop::processEvents (this=<value optimized out>, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:149
#26 0x00007f906b1fa60c in QEventLoop::exec (this=0x7fffdca7c1c0, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjuction with DW_OP_piece or DW_OP_bit_piece.
) at kernel/qeventloop.cpp:201
#27 0x00007f906b1fe6db in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1009
#28 0x00000000004088b9 in main (argc=2, argv=0x7fffdca7c958) at ../../../okular/shell/main.cpp:85

(In reply to comment #2)
> But okular can't render some pages. Seems like it stucks on rendering actually.

Correct, this is busy rendering, and we cannot do anything else than wait for it to finish; hence, I am closing this bug.
For the rendering issue, this would be another issue, and a bug to be filed for the poppler library, but ...

> Unfortunatelly, I can't provide this file as it contains private info.

... given you cannot provide a public document, a bug report for poppler would not be useful.