The PDF Extra Desktop’s compressor balances an optimized file size against the expected quality of images and other file content. If you are not sure what level of compression to use to compress PDF files offline or online, leave it to the PDF compressor and use the default optimized settings.
When choosing this setting, PDF Extra will reduce the size of your PDF files, without compromising on quality. Generally, it’s best to avoid high compression amounts unless absolutely necessary since they could significantly affect the quality of your texts and images and even make them unusable in some extreme cases.