https://github.com/botman99/Grepy2
Not something I'd describe as a replacement for dnGrep, but still simple and easy to use. Will try to search PDF files if you let it, but I'd skip that as the results are very mixed.
Steps: Download and unzip, copying all files (including those in the plugins subfolder) to the same output folder. Launch grepy2.exe
Status: Not portable, writes to AppData\Roaming\Grepy2.ini. Very few settings are saved so this one may qualify as portable enough for some.