Why pasted text hides the fault
Most free ATS checkers ask you to paste your resume into a box. By then the hard part is already done — you copied the text in the right order, by hand. The sidebar that a parser would interleave with your job history arrives tidy, and the checker grades a version of your resume that no employer will ever receive.
This tool opens the PDF itself and pulls out every text run with its coordinates, width and font size, using pdf.js. That is the same raw material an applicant tracking system starts from, so a layout fault that would scramble your document in a real parse shows up here as a finding instead of quietly disappearing.
If the file yields almost no text, that is itself the answer: the resume was exported as a picture, from a design tool or a scanner, and the system will index it as a blank document. A checker fed pasted text can never tell you this, because you supplied the words the file does not contain.