Welcome to the forum asoftwareuser794.
There are some things worth saying:
A. The version of Scribus given by Mint is not - at the time of writing - the right one. Mint 17.x gives 1.4.3 which is old and 18.x gives a weird version that doesn't have colour management. See http://forums.scribus.net/index.php/topic,2464.0.html for more information.
B. Scribus is not a PDF editor. Scribus is for creating PDFs. It can import PDFs but it should not be used for reading/editing them. Even if Scribus 1.5.x introduces some PDF editing functions they will be experimental at best. If you need to edit PDFs then use something else.
C. Scribus 1.5.x is a developmental branch of Scribus. All the 1.5.x versions are "experimental" and should only be used if you have a good knowledge of how Scribus works so that you know what is a bug from what is normal. The word "unstable" in the link you followed gives a clue.
D. Printing from Firefox to a PDF printer driver really doesn't have anything to do with Scribus.
Unfortunately when asking questions at other locations and reading articles, scribus comes up as an option, thus my usage and this question. In the "Opening Existing Document" dialog, "PDF Document" does appear in the "Files of type" dropdown. It is also mentioned in the manual, under "Importing Output Formats". I took a leap, if one can generate a PDF, they might be able to read a PDF.
I did read carefully about the version(s). I can only take what LM, based on Ubuntu, provides. The doc (snippet below) did say 1.4 and 1.5 could be installed side-by-side. I left 1.4 in place and chose, with caution, to try the AppImage.
I'm in the directory where I installed the AppImage, made it executable, and got a "Segmentation fault" when I typed:
./scribus-1.5.3-linux-x86_64.AppImage --version
I created a new file and saved it as test.sla. However I got a "Segmentation fault" when I typed:
./scribus-1.5.3-linux-x86_64.AppImage test.sla
But if I selected the "Open Existing Document" tab, selected test.sla, it opened juse fine.
The scribus man page shows it accepts filenames on the command line:
scribus [-h|--help] [-v|--version] [-l|--lang language] [-la|--langs-available] [-f|--file|--] [filename]
This may be a problem with the AppImage.... maybe not passing the filename correctly, because with 1.4 it works:
/usr/bin/scribus Document-1.pdf
Maybe the scribus appimage doesn't support the command line, which would explain the segfault with "--version".
As far as the pdf's go. I have had limited success (as you mentioned). I tried various pdf files, and by that I mean, not created by Scribus. I exported a newly created document as PDF and opened it in scribus 1.5 with no problem. So it must be how the Cups PDF printer on my linux is creating the pdf file. The Cups PDF printer must not be creating a "valid" pdf file according to Scribus.
snippet from
https://www.scribus.net/downloads/:
The development branch is currently 1.5.x. It’s mainly being released for people who want to help us testing and improving Scribus, so the next stable series (1.6.x) will work well. However, as of version 1.5.1, we consider the development branch reasonably stable, so it can be used for serious work by users who accept that not all new features are already working perfectly. Features already available in the officially stable version will work much better in 1.5.1+, though.
It is also possible to install 1.4.x and 1.5.x side by side, so you can work with one version and play with the other.
Thank you.