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PDF is not built to be responsive, it is built to display the same no matter where it is viewed.
As @alxs pointed out in a comment, there are a few features that PDF viewing applications have added to simulate PDFs being responsive. Acrobat’s Reflow feature is the best example of this that I am aware of and even it struggles with most PDFs that users come across in the wild.
One of the components (if not the only one) that matters, is that in order for a PDF to be useful in Acrobat’s Reflow mode is to make sure that the PDFs you are creating contain structure information, this would be a Tagged PDF. Tagged PDF contains content that has been marked, similar to HTML tags, where text that makes up a paragraph is tagged in the PDF as being a paragraph. A number of PDF tools (creation or viewing) do not interpret the structure of a PDF though.