If an online book image is blurry or difficult to read, it could be that your web browser did not process the PDF file correctly. Try to view the image through a different browser. Be sure that the browser is up to date.
If the image is consistently blurry across multiple browsers, please report the problem. Look on the left edge of the FamilySearch Digital Library page. Find and click the white Feedback button. Click one of the small faces then fill in and submit the form. Your report goes directly to the group that maintains the online books for FamilySearch.