Consider The Reasons Why The World Has These Elements
If your novel is sci-fi, why do they still have medieval elements in this world?
Let's consider magic first. Does the magic exist independently of technology or is it a direct result of technology?
If this is hard sci-fi, then everything in this world ultimately has an understandable real-world explanation. Dragons were probably the result of genetic modification. Wizards are actually scientists in long robes. Anything that seems like it is magic is probably just a sufficiently advanced technology that seems really impressive but has a scientific explanation behind it.
Lightning spells are just bolts of electricity, levitation spells are just tricks with magnetism, and golems or other constructs are simply robots in disguise. That's one way to take this.
Alternatively, you could have magic be an independent source of power. Advanced technology exists, but there is also a set of natural forces that are unique and totally unpredictable, and this is where magic comes from. Magicians and scientists might have no overlap, or they might even be mortal enemies. One might rely on logic while the other might rely on emotion.
So let's look at each scenario.
Hard sci-fi
The explanation for the medieval elements is that they're all the result of technology in some way or another.
Dragons and other magical creatures were created through gene splicing or similar methods. A simple explanation most people could understand.
Magicians and wizards are simply scientists holding technology so advanced that it seems indistinguishable from magic. They might wield nanobots or reality-warping technology. They can do a wide variety of amazing things, but everything they do can be explained by science at the end of the day.
As for why they would use paper, we still use paper today. Perhaps, though society is advanced, people still love simple living. They have the technology, but they prefer using paper simply as a cultural habit. They love "the olden days" and still want to use stuff from that time.
Alternatively, only the rich know how advanced technology works, while everyone else is poor. One half of the city is stuck in medieval times while the other is far into the future. That's another explanation.
Both Sci-Fi and Fantasy
If technology and magic are two opposing forces with little or no overlap, then anything that is not science can be explained by magic or vice versa.
Why are there dragons in a sci-fi setting? Because this is also a fantasy setting. Dragons are magical creatures that lived in this world long before technology was built.
Why do people use magic? Because, while technology is great, it can't do everything and there are still people clinging to the old traditions. Perhaps magic is a sort of life force. Everybody has a little bit of it inside them.
Scientists might try to understand magic and find it's not that different from technology. Or they might just be stumped. Nothing they try can figure out how this stuff works. Only magicians truly understand it.
Why do people use paper? Because, again, some people just like the old ways better. Magic likes to keep things simple. It's easier to draw a magic glyph on paper than on a screen. In fact, magic hates being anywhere near technology.
Even if your world is sci-fi, there are tons of logical explanations for how and why certain cultural elements would keep them in medieval society.
They might be so advanced but just love the look of old things, or the rich hoard all the technology so one side lives in squalor and everyone else is in a futuristic utopia, or...Anything. Go wild with your ideas.