Technology

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

PDF to JPEG

Don't most pdf viewers have an export to image option?

AVC to MP4

Do you actually have files with an .avc extension? AVC is a codec that can be used in many different container formats, including MP4. Where did these files come from?

OPUS to MP3

I actually agree that most audio conversion tools are needlessly awkward. Audacity will convert these just fine, though doesn't really do bulk conversion. Foobar2000 will do it in bulk if you're on windows.

I can’t comment on the others, but PDF to JPEG should be easy enough. ImageMagick, which another commenter suggested, is possible but not user friendly. However you can just open the PDF in many applications and export it as an image. Adobe Acrobat and Photoshop can do it. GIMP probably too.

I’m a last ditch effort you can even just open the file and screenshot it.

You could try Permute. It's a pretty simple app for converting video and audio. Permute is my go-to for quick video conversion.

DBPowerAmp has an easy-to-use audio converter that supports pretty much every audio format and does batch file conversion as well. DBPowerAmp is my go-to for audio conversion.

Both of these are paid apps.

In Windows, Foobar2000 does easy audio file conversions, once you have installed the relevant codecs.

Video and audio conversion can be done with Handbrake or Shutter Encoder, both are nice GUIs