DocumentGrep
Search text or a regular expression in multiple documents
...You can use regular expressions for the search (https://en.wikipedia.org/wiki/Regular_expression). This GUI and the command line tools work without indexing. Either the document is converted into text and processed by the RegExpr libary of Andrey V. Sorokin or handeled by the cli command itself (like pdfgrep).
This GUI works well when searching in several hundreds of documents, depending on the speed of your system and the length of the documents.