When I save a converted file to a pdf, the metadata shows it as being unchanged. When you save an image file as a pdf and then alter it to (for example) doctor an image or change the text, and then save it again, are you telling me that the metadata will remain unchanged?
I am saying that when you work with a file, you alter the text and/or image(s) first, then save it as a new file. Easy way is to Copy All, then Create New File (pdf) and then save. Metadata shows file as Unchanged Original since it was only then created, in other words: an "original". It will not have the same metadata as for instance from your snapshot of a webpage, but when the webpage is updated it would also change the metadata on a saved file you snapped later.
If they updated the webpage after you created your "original" time-and-datestamped pdf file, your creation would appear to be more original than the updated webpage since your creation point would be before the dated page change. As well as your created pdf having metadata showing it as unchanged from the creation point.