tiff to txt conversion is usually related to text recovery attempts from picture scans, faxes or similar images. For this you will have to use some of the dedicated OCR (optical character reader) utilities or online website services. Googling for "free OCR software" yields plenty of viable options, such as the popular FreeOCR or Free Online OCR.
Articles and other information relating to tiff into txt conversion:
How to quickly get text from image - Recoverting texts from captured pictures using OCR