I have several computers with the exact same programs. Okuar crashes on my home computer when I try to open a pdf file that is digitally signed. I can open the same document on all my other computer with no problem. To be clear, I can open a regular pdf file without any problem. I only have this issue on one out of 4 computers with the identical programs. I even uninstalled, clear the cache and reinstalled Okular and still crashes when I try to open a digitally signed pdf document.
Here's the developer information:
Executable: okular PID: 4768 Signal: Segmentation fault (11) Time: 4/23/19 19:58:16

Application: Okular (okular), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f63b47bb200 (LWP 4768))]

Thread 5 (Thread 0x7f6384d37700 (LWP 4774)):
#0 0x00007f63ad1d79f3 in futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x5555c27683f8) at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1 __pthread_cond_wait_common (abstime=0x0, mutex=0x5555c27683a8, cond=0x5555c27683d0) at pthread_cond_wait.c:502
#2 __pthread_cond_wait (cond=0x5555c27683d0, mutex=0x5555c27683a8) at pthread_cond_wait.c:655
#3 0x00007f638a75de6b in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so
#4 0x00007f638a75db97 in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so
#5 0x00007f63ad1d16db in start_thread (arg=0x7f6384d37700) at pthread_create.c:463
#6 0x00007f63b043a88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 4 (Thread 0x7f6385679700 (LWP 4773)):
#0 0x00007f63ad1d79f3 in futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x5555c298816c) at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1 __pthread_cond_wait_common (abstime=0x0, mutex=0x5555c2988118, cond=0x5555c2988140) at pthread_cond_wait.c:502
#2 __pthread_cond_wait (cond=0x5555c2988140, mutex=0x5555c2988118) at pthread_cond_wait.c:655
#3 0x00007f638a75de6b in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so
#4 0x00007f638a75db97 in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so
#5 0x00007f63ad1d16db in start_thread (arg=0x7f6385679700) at pthread_create.c:463
#6 0x00007f63b043a88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 3 (Thread 0x7f639b756700 (LWP 4771)):
#0 0x00007f63b042dbf9 in __GI___poll (fds=0x7f6394004db0, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1 0x00007f63aaea0539 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x00007f63aaea064c in g_main_context_iteration () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3 0x00007f63b0d7015b in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#4 0x00007f63b0d1164a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5 0x00007f63b0b3941a in QThread::exec() () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#6 0x00007f63b11de015 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5DBus.so.5
#7 0x00007f63b0b3abc2 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#8 0x00007f63ad1d16db in start_thread (arg=0x7f639b756700) at pthread_create.c:463
#9 0x00007f63b043a88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 2 (Thread 0x7f63a4377700 (LWP 4769)):
#0 0x00007f63b042dbf9 in __GI___poll (fds=0x7f63a4376cb8, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1 0x00007f63adc53747 in ?? () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
#2 0x00007f63adc5536a in xcb_wait_for_event () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
#3 0x00007f63a58f232a in ?? () from /usr/lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#4 0x00007f63b0b3abc2 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5 0x00007f63ad1d16db in start_thread (arg=0x7f63a4377700) at pthread_create.c:463
#6 0x00007f63b043a88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7f63b47bb200 (LWP 4768)):
[KCrash Handler]
#6 0x00007f637f882714 in SECMOD_ReferenceModule () from /usr/lib/x86_64-linux-gnu/libnss3.so
#7 0x00007f637f882c8c in ?? () from /usr/lib/x86_64-linux-gnu/libnss3.so
#8 0x00007f637f882d30 in SECMOD_AddNewModuleEx () from /usr/lib/x86_64-linux-gnu/libnss3.so
#9 0x00007f637fd3b8fe in SignatureHandler::init_nss() () from /usr/lib/x86_64-linux-gnu/libpoppler.so.73
#10 0x00007f637fd3bc99 in SignatureHandler::SignatureHandler(unsigned char*, int) () from /usr/lib/x86_64-linux-gnu/libpoppler.so.73
#11 0x00007f637fc5dc25 in FormFieldSignature::validateSignature(bool, bool, long) () from /usr/lib/x86_64-linux-gnu/libpoppler.so.73
#12 0x00007f63840dad55 in Poppler::FormFieldSignature::validate(int, QDateTime const&) const () from /usr/lib/x86_64-linux-gnu/libpoppler-qt5.so.1
#13 0x00007f63840db19f in Poppler::FormFieldSignature::validate(Poppler::FormFieldSignature::ValidateOptions) const () from /usr/lib/x86_64-linux-gnu/libpoppler-qt5.so.1
#14 0x00007f6384329240 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#15 0x00007f6384323626 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#16 0x00007f6384323ade in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#17 0x00007f6384323edf in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#18 0x00007f638432408a in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okular/generators/okularGenerator_poppler.so
#19 0x00007f6399ce105a in ?? () from /usr/lib/x86_64-linux-gnu/libOkular5Core.so.9
#20 0x00007f6399ce16fd in Okular::Document::openDocument(QString const&, QUrl const&, QMimeType const&, QString const&) () from /usr/lib/x86_64-linux-gnu/libOkular5Core.so.9
#21 0x00007f639a00e6db in Okular::Part::doOpenFile(QMimeType const&, QString const&, bool*) () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#22 0x00007f639a00ed27 in Okular::Part::openFile() () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#23 0x00007f63b4399a56 in ?? () from /usr/lib/x86_64-linux-gnu/libKF5Parts.so.5
#24 0x00007f63b439a8e1 in KParts::ReadOnlyPart::openUrl(QUrl const&) () from /usr/lib/x86_64-linux-gnu/libKF5Parts.so.5
#25 0x00007f639a000e06 in Okular::Part::openUrl(QUrl const&, bool) () from /usr/lib/x86_64-linux-gnu/qt5/plugins/okularpart.so
#26 0x00005555c0d4eae2 in ?? ()
#27 0x00005555c0d4ed72 in ?? ()
#28 0x00005555c0d48306 in ?? ()
#29 0x00005555c0d47955 in ?? ()
#30 0x00007f63b033ab97 in __libc_start_main (main=0x5555c0d472c0, argc=2, argv=0x7ffd5592f0a8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffd5592f098) at ../csu/libc-start.c:310
#31 0x00005555c0d48fda in _start ()

My System Info:
System: Kernel: 4.15.0-47-generic x86_64 bits: 64 gcc: 7.3.0
Desktop: KDE Plasma 5.15.4 (Qt 5.12.0) Distro: KDE neon User Edition 5.15

Evidently this is a bug with Okular. On one of my work computers, the same digital pdf file opened up perfectly until I updated the system. Then it crashes. I'll report this as a bug.