Seems like I'm a bit late to this particular party, but'll chime in nonetheless -- in case someone needs this in the future.
The best option, as mentioned, is to re-vectorize the raster image (but the source is pretty low quality to begin with).
If the image in the PDF is a vector image (and the one in the OP does looks like that), then no, re-vectorization is not the best option, at all.
You don't need Adobe Acrobat to edit PDF files. There are great freeware solutions for that, the best of which is Inkscape (https://inkscape.org/en/
). BTW, PDF files have almost the same format as Adobe Illustrator files, AI; it is even almost always possible to view an AI file while not having anything other than Adobe Acrobat Reader installed -- by simply renaming AI to PDF (that is, changing file extension from .ai to .pdf) and opening it. What I'm saying is that PDF is not some read/view-only format, you can mess with it quite a bit if you know how...
So, one way is to open that PDF in Inkscape, remove everything except that diagram, and save it as a PDF, AI, SVG or a ton of other vector formats that Inkscape supports.
If that fails, for whatever reason, then you can still at least render that particular page of the PDF, using Inkscape's command line tools, into some very high resolution bitmap and then use that.