If all of these tools do not find the text, then the text is not there.
Note that a PDF may consist merely of graphics of the page. The graphic represents the page which you can read, but it is not the actual text that is contained in the file. Instead, the PDF file contains images of the page. In that case, neither the PDF program nor other tools will find text because the actual text characters are not there.
Such PDF files are typically created by scanning software. The scanner creates a digital image of the page. Because graphics, not text, are in the file, the text cannot be found.
Such files can be made to be searchable by optical character recognition. With that process, the text on the graphics is converted to real text, which also is stored in the file, making it searchable. When finding text, the software searches in the added, real text while the user sees the graphics representation of it.
The best quality PDF contains actual text, used to render the text on screen with the dedicated fonts, and images to represent images in the text. These are typically created from application software, e.g., when exporting a Writer word processing file to PDF. These files render in very good quality, and are at the same time much smaller than PDF files created using scanning software.
