The reading area intercepts the right-click, so your browser's menu never opens over a page. That is deliberate, and it applies to the whole document area. Right-click works normally everywhere else on the page, including the details beside the reader.
What you are looking at is not the PDF
A publication is not handed to your browser as a file. Each page is rendered ahead of time and delivered as a separate image, sized for the screen you are reading on. So even if you could reach one of them, what you would get is a screen-sized picture of a single page, not the document, and not at the quality the author uploaded.
For the same reason the text on a page cannot be selected or copied. It is part of the image, not text on the page.
Use the download or print buttons
Both sit in the reader's toolbar, and both lead to the actual file rather than a screen image. Whether they are available to you depends on the publication and on your membership: where they are not, the button tells you a UNI Membership is required. The author of a publication always has both.
The download guide below sets out exactly who is allowed to download what.
If you are citing a page
You do not need the image. Cite the publication by its title, author and page number, and link to its address on UNI. Anyone following the link reaches the same page you did.
If you need the original from another member
Ask them. An author can send you a copy directly, and can ask the UNI team to open downloads on a publication of theirs. Nothing in the reader is a substitute for that, and working around the block is not something we can help with.