Under the File menu, choose Document Setup and ensure that the dimensions are correct.
If needed, change the document size and move the ad file to the middle of the new document.
Ensure that the trim and bleed sizes are correct while you have the file open.
Ensure that all marks are correct and that they are outside of the image area.
Check The Colors Used
Select the Swatches Pallete in Illustrator and ensure that all colors not intended to separate as a spot color are set to separate as a process color equivalent.
All colors not intended to separate as a spot are defined as CMYK and the Color Type is Process.
Scroll through the colors and see if the intended spot colors are set to spot and the intended ones are converted to process.
Double-click on a color to edit it.
Save The PDF File
Under the File menu, choose Save As...
Select Adobe PDF in the Format option, as shown below.
Name your PDF file and click Save.
Once you have named your PDF file, you will then be prompted to select the PDF settings.
Select [PDF/X-1a:2001] from the Adobe PDF Preset option, as shown below.
Simply click on the Save PDF button.