Because MS-DOS 7.10 is considered "Abandonware" (Microsoft no longer supports or sells it), it lives on preservation sites. The current valid sources for a fixed ISO are:
The last post was from 2015. A user named FloppyWizard wrote: "The old ISO has a broken IO.SYS. I rebuilt the boot sector, replaced the corrupt CHKDSK, and slipstreamed the USB drivers. This one actually works. Link below." download ms dos 710 iso fixed