Really, there isn’t anything. As others have said, it’s a limitation of the format. Best you can usually do is convert to a word processing file and redo the formatting manually (assuming you know what that formatting was). It’s a pain in the ass if you’re trying to improve pdfs of old, out of print stuff. If it’s your own files and you just lost the original word processing files, it isn’t as bad because at least you have memory to work from.