Pinterest Inc. has confidentially filed paperwork with the Securities and Exchange Commission for an initial public offering that is expected to value the company, which operates a platform for online-image searches, at $12 billion or more as it joins a parade of hot tech startups planning share debuts in 2019.

The filing is the latest example of a rush to the public markets by highly valued technology companies including ride-hailing service providers Lyft Inc. and Uber Technologies Inc., which have both also filed confidentially...