Heyo - I saved a logo on illustrator as PDF and it looks normal when I view it on both a mobile phone and a laptop. When I save it as a PNG and view it on a laptop it looks fine but when I view it on my phone, it looks really pixelated/liney. Any help would be much appreciated]1
A pdf file or an illustrator file uses vectors, this means you can enlarge them to any size and it will look fine. A png is a bitmap file, this means its made up of individual pixels. Decide how big it needs to be (in pixels) then export to that size.
The illustrator export has some options to do this, you can also open the illustrator file in photoshop, which tells you the pixel size in the open dialog. There's a number of guides on exporting online if you search.