The Washington Post Co. announced the acquisition of The Daily Herald Co., publisher of an evening newspaper in Everett, Wash., that has been gaining circulation in the northern suburbs of Seattle.

A formal announcement by Post Co. Chairman Katherine Graham and Herald President and Publisher Robert D. Best Jr., did not disclose a purchase price for the 53,000-circulation newspaper. A newspaper industry securities analyst. John Morton of John Muir & Co., estimated the price at more than $20 million.

The Post Co., which owns The Washington Post, Trenton (N.J.) Times and Newsweek magazine, in addition to broadcast properties, previously announced plans to expand through acquisition. Trenton was the firm's first newspaper acquisition took place in 1974, when the Post Co. purchased the Trenton (N.J.) Times for $16 million.

After suffering initial losses Post lawyer and spokesman Christopher M. Little said yesterday, the Trenton newspaper now is earning "a respectable profit." Last December, the Post Co. appointed form Sacramento Union President Edward R. Padilla to a new position in charge of acquisitions in the newspaper division. Little said yesterday the Post Co. "is actively looking at" other potential newspaper properties.

The Everett Herald was founded in 1891 and has been owned by the Best family for more than 70 years. Robert Best will continue as Herald President and no changes in editorial or business management are planned, the two companies stated.

In a telephone interview yesterday, Best said his company employs about 200 persons, including a news staff of 37. He said family members decided several months ago to sell the paper, partly to settle the estate of his father, who died two years ago.

Best said his family "drew up a list of six companies we'd like to do business with . . . and he associated with." Best subsequently approached the firms about their interest in buying the paper, which has Monday through Saturday evening editions but no Sunday publication.

"The Post Co. rose to the top, was the clear choice of all family members . . . even though it was not the highest bidder," he stated.

Everett has a population of about 58,000 and is located some 30 miles north of Scattle. Washington state officials here said the Herald's circulation is strong in Southern Snohomish County, bordering on the circulation area of the Seattle dailies - The Times and Hearst Corp.'s Post Intelligencer.

Best said a daily edition of the Herald in southern Snohomish, staffed separately and published as the Western Sun edition of the Herald, has been particularly successful in the 14 years since it was begun.

Post Chairman Graham said her company is "very enthusiastic about the Herald, a growing newppaer with a fine journalistic reputation . . . The Pacific Northwest is a fast-growing and exciting part of our contry and we look forward to being them."

Best said Post Co. ownership "assures us that the Herald's tradition of service to the community will be carried on ..."