Images with large file sizes hog memory, even when they are not able to be displayed in their full resolution on the smaller screens of phones and tablets. Mobile devices are limited when it comes to resources and processing power, and optimizing your PDF documents can be very beneficial for both performance and battery. That's why it's always a good idea to downsample images and Adobe Acrobat provides a quick, easy way to do it:
Open your document in Adobe Acrobat, go to Tools and then select Optimize PDF.
Next, click on “Advanced Optimization”.
Then, in the settings select the 'Mobile' preset. It already contains adjustments that are optimized for displaying PDFs on mobile devices, but you may want to adjust the image ppi to ensure high enough visual quality.
Also, be sure to check the transparency box.
Click OK to save the changes to a mobile optimized PDF file.