Pages can be saved in modern browsers about as easily, if they aren’t actually js apps in disguise. Never had a problem doing just that for plain html pages like this could have been.

PDF can change the same as any other format. Yes, line length in an html file if not specified is left to the browser, but css exist and also there are other formats, like txt.

I guess it depends on what wrote the PDF as to how much can be edited later, but I'm constantly opening/modifying/resaving PDFs.