Which is why I've been happy to trust them for files nobody cares about, like a random audio file that I got off the Internet. And it's very unlikely they'd be able to exploit my media player.

Yes this can also happen. I should have written open source offline converter

FFmpeg and handbrake do the latter two quite handily. The latter even has a nice program interface, rather than needing commands.

ImageMagick is capable of the first. I've had it go the other way before, and I should be most surprised if it couldn't convert a PDF to a jpg.

imagemagick handles almost all image files

images ) ls
001.jpg  002.jpg  003.jpg  004.jpg  005.jpg
images ) convert 001.jpg example.pdf

ffmpeg handles almost all video files

ex ) ls
rock.mp4
ex ) ffmpeg -i rock.mp4 rock.avi

if you use gnome there's a nice little feature of the file explorer where you can just drag and drop scripts into ~/.local/share/nautilus/scripts/

for example

make a fish script (ignoring error checking for brevity here, my real script had a couple guard rails)

/#!/usr/bin/env fish
set file $argv[1]
convert $file (basename $file .png).pdf

then when you right click on a file in your gnome file explorer you can click the scripts option

and the script is right there so you can just easily convert with the press of a button

note, i crossed out some stuff that includes client names

tldr: there are so many ways to do what you need to do there's no reason to trust random websites you don't know. there's a lot of slimey people out there wanting to take advantage of people. and everybody should strive to be at least a little computer literate. the examples i gave here aren't complicated. they're simple commands

I have zero interest in learning all the intricacies of ffmpeg so I find ChatGPT to be very useful. I’ve also used it for yt-dlp for downloading videos and converting the audio to mp3. Very useful. I personally save them as bash scripts so I can just input the file name or url as a command line argument. On Mac you can also wrap your bash scripts in AppleScript if you want to make applets for these functions. ChatGPT works great for apple script as well but I’ve had to feed it source code (eg from Apple Digital Mastering applets) to ensure it writes the new code correctly. You still must know what you’re doing.

Ffmprg is remarkably powerful. If it is a picture on your screen, you can use ffmpeg to do what you want to it. Recently discovered it could convert PNG, jpeg, and webp images back and forth. To think I've only used it for video.