PDFs are a proprietary method of saving documents as jpeg images, for the most part. As such, there is no universal way to display them. If you use Preview or another non-Adobe method, the chances are less that things will be identical.
But even if you open a PDF created in Adobe Acrobat using Adobe Reader, there is still no guarantee. Acrobat has countless ways to make a document compatible, which means it has countless ways to make it incompatible; it mostly depends on what parameters the creator used (well, not "The Creator"; the document creator).
Sometimes the document tries to use fonts from the local machine, usually done to save space in the original (rather than storing images of the font characters it sends instructions to use a particular font). That lowers the odds of compatibility, especially if your local version of Garamond Bold is different from the version imagined or used by the document creator.
One of the large frustrations I encounter is creating a PDF of a Pages document on a Mac and then seeing how Windows butchers it. Colors are typically off, drop shadows are too dark, etc. My best guess is that this is not an uncommon issue, once again because compatibility is not guaranteed; it is still the wild wild west out there in some capacities, and PDFs is one of them.
It is just the world we live in. I say deal with it; it is too much trouble with little guarantee of success to reach full compatibility, if you are just trying to download a document and read it or keep it for reference. If that is not an option, you might use Acrobat to re-edit what you need changed. Acrobat is also the best tool to create PDFs to make them as compatible as possible with your target readers.
If you are a document creator and want max compatibility, strive for that in Acrobat settings and suggest that the reader use a version of Reader that is generally compatible, rather than one that is 6 versions back. If you are just having issues displaying PDFs, make sure you have the latest version of Reader for that, as well. If the author used PDF Creator or some other tool, you may have better luck with a reader program that is from that company rather than from Adobe.
