Hi Kurisu,
I checked the PDF file on Android phone with Lollipop 5.1.1 and it is searching the text in it. However if we use the same PDF file on iOS or PC it is able to do so. May I know the source of this PDF file as it application which is used to create PDF is not Acrobat.
If it is a scanned document can you please try to scan this document with Acrobat DC with Custom Scan option. Also please try to search text on Android device in a different PDF file.