specifically editing; I know good viewing apps already :"d
very preferred for it to be free and open source software (if it’s not, then please let me know)
Don’t exactly know how you want to edit a pdf, put in case you want to annotate it, Xournal++ is usually pretty decent: https://xournalpp.github.io/
If you want to edit figures and text, inkscape should work just fine (with the newest version even multi page pdfs): https://inkscape.org/
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What are you trying to do specifically?
IMO, there aren’t any good free general purpose PDF editors for Linux.
Depending on what you want you to do there are various tools that can make certain edits.
In addition to the two mentioned above, libreoffice can also do some things.
If all you want to do is fill a PDF, there are several tools that support that as well.
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Inkscape is pretty decent, depending on what you want to do.
PDFs are notoriously difficult to edit, they weren’t really designed for that.
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sometimes I had situations in which I needed to type in text into empty lines in a PDF document, so that’s the most likely scenario
though I might need to edit images in a PDF as well, because someone I know (they are rather unfamiliar with tech in general) sometimes asks me to edit black outline drawings for them (it’s for gingerbread cookie making)
I don’t even remember how I did it on Windows. I think I did it through Foxit Reader and it had like an eraser tool, or I just used a white brush, but I’m not sure (happened a while ago). Though Foxit Reader is probably closed source
Firefox can do that. It’s a built-in feature now.
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oh really o:
very cool
Inkscape can do that.
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Okular on KDE 
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I thought it was only a viewing tool, no? o:
It supports basic editing like adding notes and text. Very limited, but if you just need to fill a form, it might do the trick.
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I also use pdftk, which is great for extracting pages into separate PDFs, concatenating multiple PDFs into one, rotating pages…
It is a command line tool, but, this is a terminal-centric distro, so you may as well learn it.
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Okular is excellent.
In a wmi3 desktop context, I invoke okular as an executable by adding o:!&okular $nnn in my plugin menu (i.e., residing in my nnn shell/config script).
So, landing on a pdf file within nnn file manager, I hit the two-key sequence “:o” and the file opens in okular - which allows for editing such as highlighting, popup notes, saving as, etc.
That’s plenty of editing for my particular needs.
Alternatively, I’ll hit my nnn plugin keybind for LibreOffice (sequence “:l” via l:!&soffice $nnn in that same config file for nnn) … so in short: “:o” opens a pdf in okular and “:l” opens it in LibreOffice. Intuitive way to pick the right app for the occasion. 
(ps - there’s also :p for nnn’s preview-tabbed plugin, but that one never seems to get me any desired functionality so it’s rarely used by me - unless I need to quickly scroll through a bunch of pdf files to see which is which.)
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not me who installed a GUI package manager as one of the first things after the system installation and still uses the Welcome app to update the system and refresh the mirrors 
I was gonna comment on howt he command line is fun until I get an error that I have no idea what it means or what to do with it and I have to take time to search it
but then I realized that I do the very same thing with errors in GUI apps
Do you mean annotating or editing? Annotating is adding text, highlighting, signing etc while editing consists of adding and removing anything. Annotating is simple, you can use Firefox or Okular. For editing I use LibreOffice Draw but I find the experience sub-par.
There is nothing wrong with that.
btw Pacseek is a nice “terminal-centric” tool to manage your packages.
oh I know. I just like to make fun of how I try to make a terminal-centric system as much GUI as possible :"D
almost as if I have arrived here to purposefully break “principles” of EndeavourOS
yeah, I installed it recently, it’s pretty good! I’m surprised that I like the visual design
I’m not sure actually, but I think the document already had empty fields for entering the text. Or maybe I had to manually place text fields on top of it… sometimes I had to edit already existing text or erase it and write my own instead
Then you need an editor. The only one I know that works on Linux is LO Draw.
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