can I upload MS word document (.docx) to MongoDB in order to search strings?
Welcome to the MongoDB Community @Shlomo_Nachmani !
MongoDB does not have any built-in features to interpret document formats like Word doc/docx, so you would have to use a third party library to extract strings and metadata to save into MongoDB. Data stored in MongoDB can be indexed and searched (with the most flexible option for text search being Atlas Search).
If your use case is focused on indexing digital documents like Word files, you may be better served looking into a Document Management System (DMS) which includes support for the document formats you want to index.
However, if you have more modest (or custom) requirements you can certainly build your own solution.