SUMMARY
Poppler on KDE Neon is out of date; this has been identified as the probable cause on a complaint that some PDFs can't be opened: https://bugs.kde.org/show_bug.cgi?id=444297#c10 (which describes it as "an ancient poppler").

This was split from https://bugs.kde.org/show_bug.cgi?id=444297 where I was complaining more generally about the unhelpfulness of the resulting error message; now that we have a shareable test case and an idea of what the problem is I'm creating this bug to see about getting it fixed. There seems to be precedent for this, it looks like Poppler was updated on top of the previous LTS that Neon was built on: https://bugs.kde.org/show_bug.cgi?id=407830

STEPS TO REPRODUCE
1. Try to open this file: https://bugs.kde.org/attachment.cgi?id=144098 with the Okular that comes with KDE Neon

OBSERVED RESULT
Error dialog, "Could not open file:///tmp/test-docusign-problem.pdf"

EXPECTED RESULT
Looks like https://bugsfiles.kde.org/attachment.cgi?id=144099

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 5.23
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.88.0
Qt Version: 5.11.0-40-generic (64-bit)

ADDITIONAL INFORMATION
Relevant extracts from "apt-cache show libpoppler-qt5-1":
Package: libpoppler-qt5-1
Version: 0.86.1-0ubuntu1
Source: poppler
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Debian freedesktop.org maintainers <pkg-freedesktop-maintainers@lists.alioth.debian.org>