Usability is in the eye of the beholder. I think the answer to your question depends on the target audience and what they're used to or comfortable with. For your application, I would try to steer away from assuming anything and only make decisions based on usability testing with your target audience.
That said, I bet we can all agree that users generally do not like surprises. If the link to the PDF does something that I don't expect it to, then I lose trust. Whether or not the link will download a PDF or open it inside the browser, I'd like to be very aware ahead of time of what's going to happen if I click any given link.
Here's some feedback from my own individual personal experience with PDF links.
- If your PDF is a large file, the LAST thing I'd want to do is open it in a browser. I find opening large PDF's in a web browser to be an especially unreliable task. Often my whole browser will freeze up or become unresponsive when I try this.
- I like seeing the file-size listed next to the link so that I know if it's going to take a long time to download the thing. It's disappointing to find out that your PDF was 50MB large only after clicking the link.
- Depending on the OS, browser, and addons being used, opening PDF's (even small ones) in the web browser is a lot more unreliable than just downloading it.
- If you give me the option to preview a large PDF file in the web browser but don't have an option listed to download, I might try and preview it just to see if I can instead of doing a right click -> save as. And if/when it doesn't work and crashing my browser, then I lose trust in your page for not knowing what was best for me.
I think a standard fair implementation of PDF linking would include the following:
- download button that actually works as a download button and doesn't open the pdf in a new tab or anything that I don't expect it to do
- title of pdf as a link to the pdf such that if I click the link it'll open in a web browser/tab
- size of pdf shown somewhere
- date the pdf was last updated shown somewhere
However, reliability is usually something a business entity would strive for so it makes sense to only allow users to download. Downloading, in my experience, is leaps and bounds more reliable than opening a PDF in a web browser. So maybe that's the thinking behind their decision.
Still, I'm sure there are stylish ways to implement PDF's for specific groups of customers.
Example 1: in-page preview
You could have an image preview of the first page that could serve as a link to opening the PDF in a frame instead of a new tab. In this way, you don't have to worry about sending the user to a new tab or a new browser. It's sort of like how amazon does with "look inside" kindle ebooks, or google books, or image galleries in general.
Example 2: screenshot preview with download option
You could have one or several small screenshots, and no preview/open-in-browser option, from the PDF to give the user some brief visual explanation of what they're about to get if they decide to press the download button.