The main competitors (Double Commander, FreeCommander XE, Total Commander) support built-in search for file contents in office xml formats (docx, xlsx, odt, ...) and epub
FreeCommander XE also supports external converter xdoc2txt to search for file contents in the following formats - rtf;docx;xlsx;pptx;doc;xls;ppt;sxw;sxc;sxi;sxd;odt;ods;odp;odg;pdf;eml (in pdf format in this case there should be recognized text, not a picture)
If you need any additional information, I can write how it works