You can use the 'Find' feature in the Edit menu to search the document.

To be able to search within a PDF, it must be OCR'd first.

  Note that handwritten or blurry /messy text may not OCR well or at all.
To OCR a PDF, do this.

1. Click the Scan & OCR icon.
   1. You can find it in the right hand toolbar, or under the Tools tab found below the Edit menu
      
2. Then choose 'Recognize Text'  > 'In this file' > 'Recognize Text' near the top of the document.