Hi Emma -- I'm sorry you are seeing an issue with the quality of your PDF image.
JPG is a file type that uses lossy compression, which likely accounts for the pixelation you're seeing. Whenever you have text in an image it's best to use a file with lossless compression, such as PNG. Can you try converting your PDF to a PNG image and see if that improves your results?
(Also, I've moved this thread out of the classic board since it's a question about ArcGIS StoryMaps.)
Owen Evans
Lead Product Engineer | StoryMaps