In Portland, Ore., advertisers create a museum as a tribute to their trade

''It pays to advertise,'' says the Portland Advertising Federation - and it's backing this pitch with $115,000 to establish what is probably the first advertising museum in the world. Opening of the museum in Portland's Old Town is scheduled for spring.

Since summer, offers of advertising memorabilia have flooded the federation, coming from every section of the United States as well as from abroad.

The goal of the Portland ad fraternity is to bring about ''better understanding of advertising and [of] its place in the free enterprise system'' and to give professionals, students, and others a source for reliable information about the advertising industry.

Earlier this fall, a major gift of newspapers, including the London Observator of Dec. 24, 1683, has come from Frederic B. Farrar of Temple University in Philadelphia. He also contributed 46 publications showing the evolution of advertising - and news, as well - spanning from 1683 through 1917. That donation includes an issue of the New York Daily Tribune of 1868, carrying what was the first full-page ad ever to appear.

Another contributor sent an issue of the Pennsylvania Gazette, dated July 5, 1760, and ''Printed by B. Franklin, Post-Master.''

Mick Scott, president of the federation, says, ''We have been offered just about everything you can imagine connected with media and advertising.''

Among offerings have been several old-time printing presses, early-day radio mikes, copies of award-winning ads of bygone years, reference material, films, and other artifacts of advertising. Besides seeking examples of print advertising, the museum also wants to include transit, direct-mail, and billboard advertising.