Artifacting is caused by over-compression of the PDF file. You can tell that your artwork has artifacting if it has a stained-glass effect instead of having flat colors and crisp lines. If you have artifacting present in your PDF file, you will need to create a brand new PDF using high quality or print settings. Please see our PDF export guide for more information (https://submit.comixology.com/site/formatting/). As a rule of thumb, view your comic at 300% this will help you to identify any potential artifacting issues.
Pixelation can either be caused by over compression of the PDF file, or by upscaling. Upscaling is when you increase the dimensions of a rasterized (or pixel based) image. With rasterized images it is not possible to add pixels only subtract them. When you increase the dimensions of a rasterized image, say from 2” x 3” to 4” x 6”, the image editing software attempts to extrapolate the missing pixels. The resulting pixelation causes the artwork to appear slightly blurry and /or soft and fuzzy. Pixelation can also be identified if the line work is not crisp and the blacks appear grayish. To avoid pixelation, scale down, never up. Again, as a rule of thumb, it’s best to view your comic at 300% this will help you to identify any pixilation related issues.
Please refer to our PDF creation guidelines for help in producing your PDF file: https://submit.comixology.com/site/formatting/