Some kind of bug while saving the PDF file

[Kettela]

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

[Joely · Accepted solution]

1. Try to «flatten» the PDF (not x-4) from Illustrator or the finished PDF inside Acrobat.

2. Try to «convert» the Brush/Stroke to a Form.

3. If its printed to a Mug, a raster/pixel image (with min. 300ppi) should do it.

Good luck.

[Kettela]

Thanks, Joely. 
The first option worked perfectly well for me. But just to make sure: I flattened the PDF using the Acrobat (as u suggested). The final result was a PDF of 1 MB istead of the original 70 MB.  How is it possible? Is it still an editable PDF? (sorry for the newbie question).
About the other 2 suggestions, I always transform all strokes to fill shapes before sending it to my clients and the Raster Effect is always setted up for 1200 dpi in EPS and the PNGs in 600 dpi. 

And a curious thing is that yesterday he replied me saying the the old PDF opened perfectly fine in his friend's Macbook. Maybe there is any kind of portability between the 2 OSs? 

Anyway, thanks for your help. 

[Monika Gause]

A flattened PDF is no longer editable, at least not to the full extent.

When sending files off to print, discuss with them what exactly they need. And then save that as a new file and keep your editable source file separate.

The saving problem might be caused by the color management bug. Try and check how the color settings are set up. If it's "Emulate Illustrator 6" then change that to the color setting you need.

And please see e.g. this link: https://community.adobe.com/t5/illustrator/colour-profile-keeps-defaulting-to-emulate-illustrator/m-...