Prefer websites of established mutts (e.g., Ahobila Mutt, Sringeri Sharada Peetham), Tamil devotional forums like Tamil Bhakti Pages , or verified PDF repositories like Archive.org .
To help you identify the correct PDF, here is how the opening invocation (Phala Shruti or opening line) typically appears in a genuine Tamil version: Sri Lakshmi Narayana Hrudayam Stotram In Tamil Pdf
If you cannot find a trustworthy copy, create a perfect one in 15 minutes: Prefer websites of established mutts (e