I have some game data files in PDF, excel, and video format. Is there any way to make it secure? Through encryption? I have almost encrypted my all files in .aes encryption format but there is a huge problem i have to decrypt it each time when i have to show it to the user ! Therefore I have to place it inside file explorer and anyone can access it. How do I prevent it?
First things first: if the data is in the user's hands (stored on their device or even just displayed), they have access to it and it should be treated as such. That means regardless of what kind of encryption you employ, if they spent enough time, they can / will always be able to get it. All you can do is make it harder for them.
Now you noticed it yourself already that encrypting is already a performance issue for you. But is that really needed? Since we know that if they really want that they can get the data, we actually just need to prevent first 99% of the users to casually read the data. Storing it in a format that is easy to read (for your application) but hard for the user (file manager) is rather trivial. Instead of saving it as a common type (like .xsl, .xml, .pdf) just store it as a byte stream.
Now the harder part: what are you trying to prevent? In case you want to prevent manipulation of fixed assets (like values for stats, scores, saves), in addition to storing the file you can store a hash. On load you compare the hash of it to the file (you would rehash the file) to make sure they are equal. This does not prevent manipulation totally since if the hash is local, it can be manipulated as well. Just another layer of making it harder.
If you want to prevent theft of assets, this is even harder. The easiest way would be probably to not use any common format but have a custom developed one. If you are the only one that uses the format, it reduces the chance that others are willing to spent the effort trying to find out how to use them. Again just making it harder.
If you want to prevent the loss of sensitive data, do not store it locally at all. Keep it on your own server to make a data mining of it harder.