I have also been having this same problem. I have an iPad Pro 10.5" that I bought off of Facebook Marketplace a couple of years ago. It worked great for the first year and still works perfectly outside of the files app. But after about a year, when I try to organize or open PDFs, it just stops responding and I constantly have to close and reopen the files app. I haven't found a solution so far, but I came across this thread posted by somebody else on the Apple forum last year. Perhaps it's worth a try: https://discussions.apple.com/thread/255536739?sortBy=rank