It looks like download links, from within the group's files section, are probably just temporary links created by yahoo, that expire soon after.
If I look at a groups page with CSS turned off, it's easy to not find anything useful to read, but it turns out the actual page content is right at the end.? So, perhaps once you know where/when to expect it, you'll be able to use those pages.
e.g. for the files section with the two manuals, here:
I see this text, which is the actual page content after all the menus etc, almost at the bottom, followed by a couple of empty boxes:
"Clipboard
??? Files
??? DX7 IIFD all I have
??? Manuals
??? Owners
dx7IId-man.pdf Download dx7IId-man.pdf
4204 KB
sampler_one
Sep 16, 2018
dx7ii.pdf Download dx7ii.pdf
5151 KB
sampler_one
Sep 16, 2018
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"
?- the first occurrence of each filename is a link to download the file.
- Andy
AndyHornBlower