PDFs created by placing scanned images will convert using PDFMarkz, however, since there was no editable text in the original PDF, the InDesign document created will consist of pages with placed images and thus no editable text in the InDesign document.
in order for conversions from PDF to InDesign using PDFMarkz to have editable text in the InDesign document, the original PDF must have editable text. [and images of pages do not contain editable text.]
If you wish to extract the text out of a PDF that was created using placed images for page content instead of actual editable text, you will need to use third party software that contains an OCR Engine (Optical Character Recognition). OCR software scans images and looks for patterns that may match characters and then creates a new file or layer and stores the characters the engine detects into that file or layer. While OCR is usually not 100% accurate, in many cases it can extract a fairly accurate glob of text very close to the original content.