Currently, deleted folders cannot be recovered.
When you delete a folder, the contents are moved to your “All Documents” folder in the space where the folder was deleted. Every space has the first folder listed as an “All Documents” folder. If you tap into that folder, then “Uncategorized”, you will see those items and be able to save them to another folder or a newly created folder as well.
If necessary, you can bulk categorize your Uncategorized documents by pressing and holding on one receipt until the blue checkmark appears. Then tap on the two lighter blue checkmarks at the bottom of the screen, then Edit, then “Folders”, and you can select a new folder to move them into.