Earlier quoted context omitted.
PDFs are supposed to look the same on every computer. Webpages can’t do that yet.
I don't know how accurate that is for PDF's, but webpages are supposed to look the same, and given known compatible styling, it should be on any modern browser. Browsers are extremely consistent in content presentation, that's why webpages from early 2000s still look the same.
One of Adobe's early talking points for the value of PDF's was that they would "look the same on all systems". Of course some context is necessary. PDF first appeared in 1993. In 1993, while the internet did exist, most individuals who were not associated with a university, research lab, or govt. agency, had no access to 'the internet'.
As well, the computing world was much more diverse. One had Dos, early Windows, and various MacOS variants all coexisting, one had numerous different variants of Unix on the numerous different RISC workstations in existence. And, here was the big deal, 'documents' created on each of these systems were to a large extent incompatible with each other. In this context, 'document' should be thought of as "a file used to create paper printouts" as opposed to what we think of a 'document' now in 2018. There was some compatibility, in that Windows systems would, sometimes, read 'documents' produced by Dos based word processors, and of course the lowest common denominator, plain text file, was 'almost' compatible (line ending differences was the biggest incompatibility). But for anything more complicated, if person X created a 'document' on Dos, and they wanted person Y, using SunOS, to see a version that "looked the same", their best bet was to print their document to paper and give Y the printer output. Because if they could send the electronic file to Y somehow, chances were that Y would be unable to open it, and even if they could, there was a good chance that it did not 'look the same' (from a 'looks like the same paper printout' level of same).
PDF came about in this world where paper was still king, and Adobe's marketing of "looks the same" was really meant to be "produces the same paper printout for the receiver Y as it does for creator X". That is why, today, in 2018, that viewing a PDF still looks like one is viewing WYSIWYG of a paper printout. PDF is, quite intimately, tied to the concept that there are discrete sheets of paper that it is formatting data onto. Yes some viewers do provide an 'almost' HTML continuous scroll look, but that is done 100% in the viewer, the underlying PDF format is very paper page oriented at its core.
So, when comparing PDF intent to web page intent, the phrase "looks the same" has different meanings. For PDF, it was designed such that "looks the same" means that a paper printout looks identical to the original. And that the designer/creator has full control over the look, while the viewer has no control over the look. For web pages, "looks the same" is far less strict, and is really not the same meaning, because the web was always intended to allow the viewer much freedom in deciding how to display the HTML content, taking away the designers ability to strictly determine look and presentation. With the result that HTML data was never meant to "look the same" with the same strictness intended by PDF.