The most accessible source for a "PDF link" (albeit a proprietary e-book) is Amazon.
Provides a limited preview of Lakhmir Singh's Science for Class 6, allowing you to read several chapters. lakhmir singh class 6 science book pdf link
: You can purchase or preview the ebook version of Lakhmir Singh's Science for Class 6 on Google Play. This version allows for offline reading through the Play Books app. The most accessible source for a "PDF link"