Cannot use the Find Text Function on PDF files created in MicroStation.
It is possible to use Find function on PDF files attached into MicroStation.
Information taken from the Microstation Help:
Open the PDF.PLTCFG file in:
File > Edit Printer Driver Configuration
and alter the file as follows:
Toggle Searchable Text to ON and save settings.
It should now be possible to Find Text in PDF files created in Microstation.