Q:

PDF images included in a PDF document with XeLaTeX

I am writing a presentation in Beamer, and I am compiling using XeLaTeX. Some PDF images I include in my presentation have a weird behaviour: they show up correctly in Mac OS's Preview application, however they don't display on Adobe Reader. 
Is there a way of solving this problem?  Researching for a solution I saw that people solved this by forcing a specific PDF version.  However I am not able to do that in XeLaTeX.
Actually I don't need the images to be included as vectorial graphics, a method to tell LaTeX to include them as raster would also be perfect. 
Thanks in advance!  

A:

I found a solution.  The problem was quite obscure, but maybe some will find it useful, so I am writing it here. 
The .pdf images I was including were generated by an application which is using the default Mac OS PDF engine.  Apparently it is generating non-compliant PDF 1.7 files.  At least that's Ghostscript output when I try to process those PDFs:
**** This file had errors that were repaired or ignored. 
**** The file was produced by:
**** >>>> Mac OS X 10.11.6 Quartz PDFContext <<<<
**** Please notify the author of the software that produced this
**** file that it does not conform to Adobe's published PDF
**** specification. 

The solution is: converting the PDF images to PDF 1.4 using the following command line
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -o output.pdf input.pdf

Then ignore Ghostscript error message, and finally use the new "normalized" PDF 1.4 files in your LaTeX document.