I created a file for an advertisement to be printed on a magazine with Illustrator, exported to PDF / x-1a: 2001.
Above the main image is a gradient with blend mode multiply to make the text pop. Opening the file with Adobe Acrobat displays it correctly, while if I open the file with a preview program the gradient creates problems no longer showing the image below. We also had the same problem in print.
Don't trust Preview with your precious PDFs.
About the printing, you have to check with your service provider. Also check out: https://creativepro.com/eliminating-ydb-yucky-discolored-box-syndrome/
Preview is just not able to display PDFs correctly.
Without seeing the PDF, nobody will be able to tell you why Photoshop can't do it. Is there a Pantone color in it?
The only PDF readers which preview reliably for print and prepress work are Adobe Acrobat and Adobe Reader. Apple Preview, browser PDF readers, and other readers are meant for casual use only.
The picture on the left is open with Adobe Acrobat, and it shows how the file was supposed to be printed but in the magazine came out wrong. I'm trying to understand if it's my fault, maybe I have to rasterize the gradient before I export the PDF, or if it's the service provider's fault.
Can you not ask the service provider what they did with the file?
No Pantone, only a CMYK gradient over a jpg
Why are you using x-la?
Is your image rgb?
We'll have to wait and see, I never like to assume...