Hold up the piece of paper you felt was brighter to your computer's monitor. The document you have open on your screen is designed with a white paper background, but it looks even brighter than the paper in your hand! Wow. The white of your paper will affect the way one sees ink colors after it's printed. There's some science at play here. Paper looks the way it does because the light is reflected to your eyes. Your computer monitor is illuminated from behind, and does not reflect, but rather projects the light to you.
Reflected and projected colors simply do not look the same to our eyes. With all this in mind, and the need for precise color reproduction in printing, it is very easy for your carefully designed document to look different in print than it did when it was designed on screen. Acrobat has become an indispensable tool in the print industry, offering a complete delivery method from designer to print shop.
Are you curious how your PDF file will look on paper? You can get a glimpse well before you send your file off to the print shop. With your PDF file open, choose Advanced > Print Production > Output Preview. Near the top of the Output Preview dialog box is a check box marked Simulate Paper Color. It is unchecked initially.
If you click this box, Acrobat will dim the paper white of your document to simulate reflected white instead of your monitor's projected white. It will look WRONG. The page on your screen will look gray! But if you walk away for a bit, and come back and look at the document, it will actually seem white–just not as bright as your monitor's naturally-projected white. It's not perfectly accurate, since you already discovered that two pieces of paper you thought were both white actually looked different, but it does give you an idea of how the ink that will be spread on top of your white paper might look.