In Windows version (Win10) symbol of "Russian Rouble" is displayed incorrectly (as ?).   When installing to the default folder, you cannot change any currency symbol.  Also a problem with the display of documentation in pdf in Russian on Win10 (many #)

That's two separate bugs. I've opened bug 797047 for the PDF one.

The symbol GnuCash uses for the RUB is ₽ as you can see at https://github.com/Gnucash/gnucash/blob/maint/libgnucash/engine/iso-4217-currencies.xml#L2161. That's codepoint U+20BD, and according to https://www.fileformat.info/info/unicode/font/segoe_ui/list.htm it's not provided by Segoe UI, the default font Windows uses, though I also found several mentions of Windows supporting it so the web page might be obsolete. The Microsoft App Store has a free program, Glyph Finder, that purports to show the codepoints supported by the fonts on your system. That should enable you to confirm whether Segoe UI is missing the symbol and if so find a font that provides it.

It's far easier to set the font for GnuCash (see https://wiki.gnucash.org/wiki/GTK3) than to change Windows 10's system font, which requires a somewhat complicated registry edit (see https://www.techadvisor.co.uk/how-to/windows/change-system-font-windows-10-3680759/). 

You can also edit the symbol for currencies. Select Tools>Security Editor and check the box in the lower left corner "Show National Currencies". "Currencies" will be added to the list box. Click the disclosure arrow next to it, scroll down to Russian Rouble, select it and click Edit. Change the symbol to something that displays on your system and click OK.

(In reply to John Ralls from comment #2)
> That's two separate bugs. I've opened bug 797047 for the PDF one.

No, it is:
Bug 797044 - Russian PDF on website has '#' for every cyrillic codepoint.