Some kind of bug while saving the PDF file

Hello. I'm a long time user (sorry for any wrong setence. English is not my native language) but it's the first time that I post something on Adobe's forum.
I usually work with complex vector artworks, and since a lot of small business which make shirts, mugs, etc, use Corel Draw, I send the final files with 3 options to avoid the compatibility problem - AI, EPS and PDF. The PDF is the most guaranteed option for my clients that will later send my file to this Corel Draw users. 
Last week, I finished the art and sent these 3 options as usual (imagem 1 and 2). But my client told me that a specific part of the character is opening with a "black square" over it (image 3). I thought it would be probably the shapes with mesh tool applied or maybe the shirt texture, but when he send me, it's a part that I drew using a custom brush (fur looking) over his shoulder. 
The odd part is that the EPS is working fine for them, but 4 or 5 friends of him opened the PDFs and all of them experienced this "bug". 
I tried to export it as a PDF X-4 (2008) instead of the standard High Quality, but it doesn't work.
Does anyone have any clue of what am I doing wrong? I don't know if it's a basic thing since my knowledge about the PDF presets and standards it's not that deep - or maybe it's because I use OS Mac and they all use Windows? Anyway, I would be very grateful if I could understand this, hahaha.
Thank