...That means that anything on that page (photos, diagrams or web links) that works as a "link" will only be there as long as the linked material is still available....
This is no different from viewing the original post on the web. If a link changes, or a photo site shuts down, the links in the original post on the web won't work either, unless that page is updated with the new link.
...Unlike Rich's method, though, the PDF process does give you permanent copies of photos, diagrams and web links.
A PDF will not give you "permanent copies..." of photos or web links. The exact same situation is in play here as above where you save the web page as an html file. If a link on a web pages changes or disappears, the link in the PDF won't work either.
With regard to photos, the images on a web page are saved as part of the PDF. You cannot pull an individual image out of a PDF to save it unless you own a PDF editing program like Acrobat. And even then, you will get only the low-resolution image that was saved in the PDF. However, if you save the page as an html file, you have access to the original images in the post. If there is a particular photo you want to save, you can right-click on the photo and save it from the menu that will come up. In many cases, the actual image file is a medium to high resolution image that has been downsized to fit on a web page. That means that when you grab it and save it, you get the higher resolution image.
You cannot do any of that in a PDF.