Technically searching the textual contents of complex file types like DOC, DOCX, ODT, PDF is done using IFilters. IFilters are Windows plugins that are used to extract pure text from complex files. It depends on your system which IFilters are available. In the web you can find IFilters for almost anything.
To check which IFilters are installed on your system you can use the free SearchFilterView by NirSoft.
For version v20.40.0000 and later:
Since v20.40.0000 (2019-09-10) both 64-bit and 32-bit IFilters are supported. Make sure that either of these checkboxes is ticked:
Configuration | Other | Shell Integration | 64-bit Windows | Use 64-bit IFilters for content search
Configuration | Other | Shell Integration | 64-bit Windows | Fall back to IFilters of the other bitness
For versions before v20.40.0000:
Note that XYplorer needs the 32-bit versions of IFilters installed. Recent versions of Microsoft Office apparently always install only the 64-bit filters when installed on a 64-bit Windows so that won't work with XYplorer.
For searching text in PDFs, if it does not work out of the box, you might install the free PDF-XChange Viewer (64-bit or 32-bit version, both work fine with XYplorer).