In the past, most ads that were submitted in PDF format were at a large size (like a full page 8.5" x 11"). They looked great when viewed at that large size. However they needed to be reduced in size to fit the 300 x 300 pixel format. Once shrunk they did not look so well and were not very legible to the point that they needed to be to attract the attention of the viewer.
By submitting the actual size in JPG, GIF, or PNG formats then you will see how well your ad will look before uploading them.