This is the 4654th most frequent Russian word.
документах
"Документах" means "documents" (prepositional case, plural) in English.
Here, 'документах' is used to indicate the location within the documents where something exists or is found.
В документах содержится важная информация.
The documents contain important information.
In this sentence, 'документах' specifies the documents as the location where the action of signing should occur.
Она забыла подписать свои имя и фамилию в документах.
She forgot to write her first and last name in the documents.
This usage shows 'документах' as where information (the date) is mentioned.