title not organizing PDFs alphabetically in i books

I renamed all my PDFs so they could be organized alphabetically by subject matter. E.g all recipes start with REC then a letter designating baking, veg etc thenthe name of the recipe. On my iPad they come out alphabetically by subjects as I want but on my I phone they don’t I have sorted by title but this is not organizing them alphabetically. There is no pattern to the chaos. If I try searching REC it will come up with every thing in books that includes that order of letters. I have some in collections and they don’t come out alphabetically either. I have almost 500 PDFs in books I can’t scroll through the lot to find one.