Most modern desktop browsers do, in fact, support PDFs. So the need for downloading another tool would be limited to the select few users who don't have one of these browsers.
But users find PDFs jarring. NNGroup said in 2011 called using PDFs for online reading the second-biggest mistake in modern web design. Why?
- PDFs are typically presented in a separate reading view from a
normal web page (a PDF reading view looks like you're reading a page
or a digital book).
- PDFs are often styled differently from a web
- Other interactions with a PDF document are different, such as
scrolling (dealing with gaps between pages) and selecting/copying
- Many PDFs have a multi-column layout designed for printed pages instead of device screens, so they can require a lot more scrolling up and down than other webpages. It's a reading experience which causes users to spend more of their effort managing the tool so that they can read a document, rather than actually reading it. (I'm aware that CSS advances are now making multi-column, print-like layouts more possible and easier to use on the web.)
- PDFs often load more slowly than other web pages, whether they
have to open another application or not.
"Get Adobe Acrobat" sounds like an at least partially obsolete statement to me, considering the need today for responsive design.
With responsive design, I would add to this: PDFs are often not optimized for browsing on a mobile device. With their assumption that users' screens are large and their small text size, they typically provide a user experience comparable to being served a desktop web page on mobile.
So, I would still consider Nielsen Norman Group's comments about "PDF shock" valid. Therefore, I would still recommend some sort of warning before users are about to switch file formats from a more conventional web page or web app into reading a PDF. It may tell them, for example, to download and print the file instead of trying to read it in their browser, or to switch from a phone to a larger device for a better reading experience. PDF icons are widely recognized, so I would recommend using a PDF icon (with "PDF" in the link's anchor text) to show users that they are about to look at a PDF.
But what about users with old browsers that cannot open PDFs? These users are often less technically-inclined (laggards), so we need a way to help them without making them summon their IT support staff. They are the users likely to need the most help using a computer in the first place.
For them, I would suggest a help link that says, "Need help opening these PDF files?" or something to that effect. This could bring up a lightbox or another page telling them what they can do, depending on the device that they are using. Possible recommendations would include downloading software, downloading an app, trying another browser, or switching to a different device.
Keep in mind that the Adobe Reader is not a user's only choice in reading PDFs, just like how IE is not their only choice in browsers. A recommendation of a tool should be more along the lines of "you can use this tool, and we suggest using it" - in other words, a good starting point - than "this is the only tool that you can use / this is the tool that you must use". Of course, a corporate IT department can often add its own rulings into this.
And the help should also address the problem of mobile users being served with desktop PDFs. As always, test this recommendation with users.
By the way, Adobe Reader (not Acrobat Reader) is available in the Android Google Play store, and it is still available from Adobe for desktop machines. So it's not that much of a relic, although putting a "Get Acrobat Reader" button on your site is a little 1990s. (I'm thinking of "Best Viewed with Netscape Navigator".)