Efficient way to search list of items in a text document
I have not tried this before but I will suggest:


Get all unique items from all documents:
Create a dictionary where key is that unique item and values is list of document ids (document names) where this item is present.


Once you are done. Just search your item in the dictionary key, it will return all the documents ids where this item is present.

This algorithm is called Inverted Indexing in Information retrieval.