Preview Save As PDF Blurry

[TartanTrumpet]

I'm using Preview to save images (jpg and png) as PDFs, but the resulting PDF is very blurry. Is there a fix for this?

 

[chabig]

All Preview does is encapsulate the images into PDF files. If the images are low resolution, they'll be blurry when they're put into PDFs.

 

[TartanTrumpet]

The images are screen grabs so will be low resolution. I've tried Photoshop; however the resulting PDFs are still blurry. What should I be using/doing to get PDFs that are nice and sharp?