What is the best way to add a second monitor to a PC with the monitor about 25' away from the PC ??
Is this second monitor for displaying a different screen than the primary?
For $30, you can get a fairly cheap graphics card with two heads, usually, one DVI and one VGA. They usually come with a DVI converter. By default, they will display the same screen. Use one of those "baluns" and Cat5 cable to ship the signal 25 feet with minimal loss and fuzzyness.
They do make heavy shielded VGA cable, probably up to 25 and 50 feet, but I wouldn't expect it to producae a terribly clear picture.
If you have no fire code issues and can run a VGA or DVI cable, then I would suggest buying the appropriate length from
https://www.monoprice.com. If you have to go into a rated ceiling and need a plenum rated cable, VGA would be too expensive. Then you could get the baluns suggested by Rob. I use these.....
https://muxlab.com/products/ve_vga_balunII.html They are not that cheap either!
another way to go, (if it's just a second view of the same desktop) is use a second PC, with VNC to capture the screen. Then standard networking between them, wired or wireless. VNC has an option to prevent the viewer from making input changes. The second PC doesn't need to be terribly sophisticated, as old as a P2 would probably suffice.
They have VGA splitters. Also, I just installed a vga cable which was over 100' in length.
https://www.national-tech.com/specs/vga-splitter-cable/10h1-27708.htm