Steps to reproduce:

1. Install the Firefox Flatpak from Flathub (flatpak install firefox, app/org.mozilla.firefox/x86_64/stable)
2. Launch the Flatpak Firefox and open https://publikations-plattform.de/
3. Press Ctrl+P and save to PDF.
4. View generated PDF in Firefox or another PDF viewer (e.g. Okular).

Another website where this happens is https://ebay.de/

Actual results:

Some fonts in the resulting PDF are garbled / pixelated.

I have attached a zip file with four PDFs and two screenshots.
The PDF files with _flatpak in the name were generated with the Flatpak version 111 of Firefox and the ones with _non_flatpak in the names were generated with version 111 downloaded from the Firefox website directly.

I have also attached two screenshots which also showcase the issue, viewed in both the Flatpak Firefox and Okular (a PDF viewer).

Expected results:

Fonts should not be garbled / pixelated in the resulting PDF.

The Bugbug bot thinks this bug should belong to the 'Firefox::PDF Viewer' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

I've been further testing on different distributions in a fresh install with the default desktop configuration and found the following.

The issue is present on the following distributions:

Debian 11 (KDE) (My currrent OS).
Debian 11 (Gnome)
Debian unstable (KDE)
Debian unstable (Gnome)
Linux Mint Debian Edition 5 Elsie (Cinnamon)
OpenSuSE Leap 15.4 (KDE)
OpenSuSE Leap 15.4 (Gnome)
OpenSuSE Tumbleweed (KDE)

The issue is NOT present on the following distributions:

Linux Mint 21.1 (Cinnamon)
KDE Neon User Edition 5.27
Ubuntu 20.04
RHEL 9.1 (Gnome)
Fedora Workstation 37 (Gnome)
Manjaro Linux (KDE)

Note: On all of these I've tested, Firefox outside of the Flatpak does not have this issue.

It'd be good to know whether this is due to font access / flatpak sandboxing shenanigans, or something else...

~/Downloads/firefox_pdf_font_bug (1) pdffonts ebay_flatpak.pdf

name                                 type              encoding         emb sub uni object ID
------------------------------------ ----------------- ---------------- --- --- --- ---------
QKWRYW+CairoFont-0-0                 Type 1C           WinAnsi          yes yes yes    122  0
OPDHEH+CairoFont-1-0                 Type 1C           WinAnsi          yes yes yes    123  0
QUXEGK+CairoFont-2-0                 Type 1C           WinAnsi          yes yes yes    124  0
[none]                               Type 3            Custom           yes no  yes    126  0
[none]                               Type 3            Custom           yes no  yes    127  0
LVICVC+CairoFont-5-0                 Type 1C           WinAnsi          yes yes yes    139  0
[none]                               Type 3            Custom           yes no  yes    686  0

 ~/Downloads/firefox_pdf_font_bug (1)  pdffonts ebay_non_flatpak.pdf 
name                                 type              encoding         emb sub uni object ID
------------------------------------ ----------------- ---------------- --- --- --- ---------
HAUBFC+CairoFont-0-0                 Type 1C           WinAnsi          yes yes yes    130  0
OFNKHT+CairoFont-1-0                 Type 1C           WinAnsi          yes yes yes    131  0
QUXEGK+CairoFont-2-0                 Type 1C           WinAnsi          yes yes yes    132  0
SQGVBO+CairoFont-3-0                 Type 1C           WinAnsi          yes yes yes    134  0
DMOMER+CairoFont-3-1                 CID Type 0C       Identity-H       yes yes yes    135  0
YDDGGM+CairoFont-4-0                 Type 1C           WinAnsi          yes yes yes    144  0
GOSFIF+CairoFont-1-1                 CID Type 0C       Identity-H       yes yes yes    431  0

Looks like some fonts are missing on the system for your files. As noted above, unable to reproduce with Fedora 38.

I am facing this issue too. Firefox 114.0.1, openSUSE Tumbleweed (GNOME).

(In reply to Emilio Cobos Álvarez (:emilio) from comment #3)

It'd be good to know whether this is due to font access / flatpak sandboxing shenanigans, or something else...

I allowed Firefox flatpak to access both /usr/share/fonts and ~/.local/share/fonts directories. But this issue remains the same.