Personal and Professional Practice 2/ 10 Minute Presentation

I'm unsure why, but while I was decreasing the file size of my PDF to make it accessible on 'Issuu', different parts of my feature designs began to look distorted and as a result the joins on some of the sections don't look perfect. However this is not the case in the original file, as you can see from the screenshot below...
Moreover, some of the larger images also look distorted when they're crystal clear within my own file. This has proven that I should seek alternative methods to uploading or different methods of decreasing file sizes