I currently use Reader by Readwise for this purpose. 3 main things I look for in such an app are:
1. Ability to save everything including PDFs etc.
2. Ability to read offline
3. Some ability to summarize, send some reminders to read etc. are an added bonus.
I'm not sure on the support for PDFs with hoarder-app, the github README doesn't seem to mention anything about it.
My problem with using Firefox is the extensions. Though one of my faves is Firefox exclusive(Yes, Downthemall!), but some of the other useful extensions(like Google Dictionary and it's alternatives for Firefox are crap) are not available for Firefox and that often pulls me back to chrome after some time.
I'm not sure on the support for PDFs with hoarder-app, the github README doesn't seem to mention anything about it.