Okay,now I will tell you guys the simplest way to print EAGLE PCB in negative mode. This method does not require you to buy anything from any website or any shop or use any editing software. COOL HUH!!
1. Download and install Ghostgum GSview(v4.9 or latest) from any available website.
2. In eagle board editor, click CAM.
3. In the popup window, Output section, select "PS_INVERTED" for Device, type "1" for Scale, click on File and save your file with extension ".ps".
4. Highlight the layer you want to print on the right, and you can choose whether to fill the pads or not by ticking it in the Style section(optional).
5. Click Process Job and the file will be created in a Postscript format in the folder you've chosen.
6. Open the file with Ghostgum GSview(v4.9 or latest).
7. In GSview, click on File>Convert...
8. Select "pdfwrite" for the Device, select the highest Resolution and Fixed Page Size for the column under the Device column.
9. Click on Properties, for CompatibilityLevel select "1.5", for PDFSETTINGS select "/printer" and set all other to "[Not defined]".
10. Click OK, then click OK,save the file in extension ".pdf".
Your finished. You will have a negative mode PCB with perfect resolution in no time.