Hello Tomek,

Searching in custom fields should work, please follow steps below:

1. Please create new field (multiple choice in your case, e.g. value;name) for your document type in Fields tab.

2. Please switch to 'Search fields tab' and set this new fields as Content, Searchable and Tokenized.

3. Please rebuild search index in: Site Manager -> Administration -> Smart search. Whether you have more indexes, please make sure you are rebuilding correct one. Optionally please restart system (just to make sure ;-)

4. Now, if you have it checked in Form tab of your document, it should be searchable - when you will search for a value, you should receive document in your search results.

Hope it will work well.

Best regards
Ondrej Vasil