As others have said, you would be wise to install an outlet near
the AC location. Then there is no concern about tripping over an
extension cord, damaging the cord with furniture, somebody
tossing a rug over the cord, etc.
However, you can purchase a good quality UL listed, 12/3, 25-foot
extension cord for $20. That will handle every one of the 15 amps
that your standard household outlet will provide. Why does the
Underwriters Laboratory approve the 12/3 25-foot extension cords,
rated at 15 amps? Because those experts at testing electrical
devices, have determined that the cords (and associated insulation,
plug, and socket) safely do what they are supposed to do - provide
15 amps 25 feet away.
Just don't be stupid. Use some common sense and avoid running
the cord under a carpet, stapling it to the baseboards, unplugging it
while the AC is running, putting it where it could be a tripping hazard,
allowing the pet rabbit to gnaw on it, saving money by buying a $5
16 gauge cord at the local outlet store, etc.
There are many, many accidents and fires every year due to extension
cord use. There is always a moron user associated with each of those
sad situations. That is the reason why there are so many warnings
in the owner's manual for any appliance with a reasonable current
draw. There is always some moron who needs a simple "don't use
an extension cord" because otherwise he's going to use that 90 foot,
2-strand, 32 gauge, 1910 vintage cord that great grandpa left him.
Those warnings must be there for the same reason that we are advised
many times in an owner's manual not to use the electric hair dryer
will taking a bath or a shower. Etc.
If you still feel a bit concerned, then buy a commercial grade 12/3
25 foot cord for $40-$50. You'll be impressed with the over-engineering
that goes into a better quality extension cord.
I'm certain that there is a wealth of advise available from Google on
prices, caveats, etc.
One final time: If you are smart and prudent, then extension cords are
very safe. If you are dumb or careless, then the gene pools really
doesn't need you anyway. :)