That will create a single JPG with two columns and as many rows as needed to include all the pages of the PDF. The -geometry and -density params should make the quality of the output pretty good. There will be nothing fancy — no background, border, shadow, etc. Save that for later after you get the basic conversion working to your satisfaction (as shown in the last image of my article, if you want it fancy like that).
gm.exe montage -tile 2x10000 -geometry 100% -density 200 input.pdf output.jpg
Most are online. The above is downloadable (so I assume local).
There is no native capability in my Adobe Acrobat DC to create JPG files.