Illustration export to PDF - Got an issue

Hi everybody,

I'm at work and I had to create an illustration based on an old illustration.

It is an illustration made with lines. I created it on the same size (2000x1200mm).

When I export it in PDF file, the result is very bad. I can't show it because it is highly confidential, but I took some screenshots to show you :

(img1 : the original file in PDF)

(img2 : my illustration in PDF)

(img3 : my illustration in Illustrator)

On the screenshot, the difference is little because my screenshots are in a bad resolution when I upload it on the forum. But when I see my entire drawing, it is very very bad and the differences are huge.

Do you know what I mean ?

How can I export it in a high resolution ?

In my parameters, everything is on maximal resolution...

Thanks by advance