How do I explain to my students how to download less pixelated images?

I teach a content creation course at a high school.  My students use Adobe Express to create things like social media posts, logos, etc.  They always look great in Adobe Express, but once downloaded (as .pdf or .png) they are super pixelated and unusable.  We have tried changing every setting I can think of.  How do I help my students download their creations in a format that matches the effort and creativity they have put into their designs?