The best way to extract the images from your pdf is to keep them in their original format and quality as they have been saved in the pdf. Later you can still convert them in Photoshop or so for optimal results.
For this image extraction, I recommend you to have a look at Extract images from PDF Service. It's online and free and extracts all your images in their original format.
You can try PDF library. It supports Mac OS X 10.10, 10.11 and 10.12. Open your PDF. Then click Convert and select To Image from the submenu. When the right side options expanded, you can choose JPG from the Format list and chose Keep Original DPI from the Image Quality list. Finally click Convert to save PDF as JPG while maintaining the original quality.
Not to sacrifice the image resolution, you will need advanced PDF tools to perform the conversion. No matter you want to convert PDF to JPG on Mac or on Windows PC, you can try following:Adobe Acrobat: the most powerful PDF tool to manipulate PDFs
Try a PNG instead? It's lossless and widely supported. It compresses documents (as opposed to images, where it varies more) very well.
I've used Photoshop successfully for doing this conversion in the past.
An alternative that might work for some use cases would be to e-mail the PDF as an attachment to a Gmail account. Gmail usually will let you preview PDF attachments as images, and maybe you could save one of those.
You may choose from several fantastic websites! Convert anything into anything, with 1 click is by far the best online converter I have come by. It has such a great interface and does the job in No Time.
To save PDF to images for easily sharing, viewing and others, PDFelement is what you need. It converts PDF pages into images like JPG, JPEG, PNG, BMP, TIFF, etc. What's more, the images in PDF file can be exported to your computer.
You are somewhat confused in that PDFs don't naturally have a resolution, and when you export the JPEG image from Preview, you specify a target resolution -- so the image resolution, by definition, can never go down.
I suspect that you are complaining about the quality of the image the you get when exporting from Preview. There are two parameters that Preview allows you to set when exporting a PDF to JPEG: resolution and quality.