Reading order is a guess
Text extraction pulls each glyph the PDF draws, along with where it was drawn, and reassembles it. For a single column of prose that works almost perfectly. For anything else it is an inference: a two-column academic paper, a newsletter with a pull quote, or a form with labels beside fields can all come out interleaved, because the order the glyphs appear in the file is the order the generator happened to emit them, not the order a person reads them.
Tables suffer worst. Most PDFs store no notion of a row or a cell — only text at coordinates — so a table arrives as a run of values with the grid that made them meaningful gone.
Empty output means a scan
If nothing comes back, the PDF has no text layer: the pages are images. That is normal for anything scanned, photographed or exported from some fax and signature systems, and it is the single most common surprise with this kind of tool. Extracting words from a picture requires OCR, which is a different technique entirely.
This tool is also the quickest way to find out which kind of PDF you have before trying PDF to Word or PDF to Markdown. Once you have the text, Word Counter and Find Frequent Words will analyse it.