If you need to open PDF files or just want a decent image editing app that doesn't cost too much or require a subscription, there are plenty of options in the Mac App Store and elsewhere. All of these and 64-bit apps that work with macOS Catalina and beyond. See https://macmost.com/mac-photoshop-alternatives.html for links.
You can create custom folder icons by using only the software that comes with your Mac. By taking the existing folder icon graphic and bringing it into Preview, you can tint the color and then add text, emoji or shapes to create a useful custom folder icon. Changing folder icons can be a little tricky and buggy, however. You can also use graphics apps to create more highly-customized icons and download icons online to use.