Testing a PDF for Reflow

Hi all.

I am looking for some guidance on exactly how to test for Reflow in a PDF document. I always assumed it should be tested in Adobe using the Reflow option rather than the browser.

Adobe has the following guidance, but I am still not 100% sure after reading the  advice below:
Reflow a PDF
You can reflow a PDF to temporarily present it as a single column that is the width of the document pane. This reflow view can make the document easier to read on a mobile device or magnified on a standard monitor, without scrolling horizontally to read the text.
You cannot save, edit, or print a document while it is in Reflow view.
In most cases, only readable text appears in the reflow view. Text that doesn’t reflow includes forms, comments, digital signature fields, and page artifacts, such as page numbers, headers, and footers. Pages that contain both readable text and form or digital signature fields don’t reflow. Vertical text reflows horizontally.
Acrobat temporarily tags an untagged document before reflowing it. As an author, you can optimize your PDFs for reflow by tagging them yourself. Tagging ensures that text blocks reflow and that content follows the appropriate sequences, so readers can follow a story that spans different pages and columns without other stories interrupting the flow.
To quickly check the reading order of a document, view it in Reflow view.
(Acrobat Pro) If the tagged PDF doesn’t reflow the way you want, see if the content order or reading order of the PDF file contains inconsistencies. Also check the tagging process. You can use the Content pane or the Reading Order tool to resolve reflow problems.

Source: https://helpx.adobe.com/acrobat/using/reading-pdfs-reflow-accessibility-features.html#reflow_a_pdf

Adobe themselves say "...and the layout and formatting will always be consistent with the original file." which sort of suggests it's a fixed view, which is the point of PDF, so it seems to me like reflow is not relevant for PDF?
Any help is appreciated!

Thank you.