Helping people with computers... one answer at a time.
When a PDF is printed, even though it may be standard sized pages, the printer may add margins. You can adjust this behavior when you print.
Why do my PDF's print out smaller than the original copy?
I'm going to assume you mean slightly smaller, since that's a very, very common scenario. I see it myself all the time.
The answer, in a word, is margins. And depending on the PDF document and the capabilities of your printer, you may, or may not, be able to work around it.
Most PDF documents are intended to be a representation of an equivalent paper document. That means that they're typically sized as if they were paper. That means they usually represent a paper size 8.5 by 11 inches (or perhaps A4 sized paper in Europe).
When you view the document in a PDF reader, it provides a view on those series of pages, but they're still conceptually pages.
Now, when you print your document, it would seem logical that an 8-1/2 x 11 page would print just fine on an 8-1/2 x 11 piece of paper. Sadly, that's typically not the case.
The problem is that most printers cannot actually print on the entire sheet of paper. There's a "margin" around the outer edge that will vary depending on the printer. It's usually around 1/4 inch (but I must emphasize that this varies greatly depending on the printer). The upshot is that a printer will only be able to print on an area somewhat smaller than the full 8-1/2 x 11.
Now, when printing there are two choices:
You can print your 8-1/2 x 11 document directly on to the 8-1/2 x 11 paper, and risk cutting off from view the edges of whatever is presented in the PDF.
You can resize the 8-1/2 x 11 document down to a size that will actually fit entirely in the printable area of the paper.
The later is the most common default, and it's likely exactly what you're seeing.
Before and after - ever so slightly smaller to fit in printable area.
The thing to look for in your PDF viewer is "Zoom" or "Page Scaling":
As you can see the default is "Fit to Printable Area", exactly as I've described. (And you can see a "Zoom" of 97% has been applied just under the preview.)
Now, the example page I'm using would actually print just fine without being resized smaller (since it actually has margins of its own).
Simply change the "Fit to Printable Area" to "None", you'll see the "Zoom" disappear or reset to 100%, and the PDF reader will attempt no scaling - it'll just print the 8 1/2 x 11 document onto 8 1/2 x 11 paper. If there's clipping because of the printer margins, so be it.