Split by File Size not splitting correctly

I am trying to break a pdf file into smaller documents. I select to split by file size and set it to 8mb, but it creates 3 documents, one that is 2mb, one that is 3mb, and one that is 10mb. When I chose to split file size by 5mb it again creates 3 documents of those same sizes. is there a fix for this? I don't care how many different files it needs to create, just so long as none of them are larger than 8mb.