There are a few ways to determine the order of documents within a folder in the Library:
The first is by setting the sort order of a folder, which can be alphabetically by title, reverse alphabetically by title, by create date newest first and by create date oldest first. You can set the sort order by editing a folder or when you create a new folder.
If the folder sort order setting is set to alphabetical, the best way to manually control the order of documents is to add numbers to the document titles, like so:
01. Some document
02. Another document
03. A third document
Unless you know you're going to have less than 10 documents in the folder, make sure the number includes the leading "0" to keep the correct sort order.