If you click on the printer icon at the upper left of the PDF, a menu will pop open. There are familiar print options, letting you select pages and decide how many copies you want, and then there are some additional ones. In particular for the PDF print menu, there is an drop down box called page scaling.
Page scaling in the Adobe reader has five choices:
None: This is obvious- you are keeping the image at the size it appears at 100%, because it fits on a paper size in your printer. Here we are using the choice current view for the next few options.
Fit the Printable Area: This puts the page from the PDF onto the paper using as much of the paper as it can. It will keep the same proportions as the original 100% view. This can have the effect of enlarging your image, if it is smaller than your paper. If the PDF is larger than your paper, the PDF will be scaled down to fit onto the paper. This can have the effect of making images much smaller than the 100% view, and can make print difficult to read.
Shrink to Fit: When your PDF is larger than your paper; If you have a 10 x 13 page, and 8.5 x 11 paper, this will also make the image smaller to fit in the printable area on the page. In this case, since the current view is selected and is smaller than the size of the paper, it appears at 100%, and is identical to page scaling none.
Multiple Pages Per Sheet: This will put as many pages on a sheet as you select. You can give custom settings or use the multiples of 2 up to 16 that are preset. Sometimes using a custom setting to put the same 2 pages on a sheet makes the pages larger because they are more efficiently placed.
Booklet: This will print a document which is printed on both sides of the paper and is set up to have pages printed so the fold is in the right place.
If your default is set to Fit the Printable Area, you may get zoomed prints if the actual size of the PDF is smaller than your paper. If Fit the Printable Area is your default, you may not know what you are likely to have printed unless you are careful to keep an eye on print preview.
If your default is Shrink to Fit, you may get PDFs printing with scaled down text all the time. This can be frustrating if you constantly have PDFs which are larger than 8.5 x 11 or 11 x 17 paper, because they will shrink to where the text cannot be resolved. The answer is to check your default print settings, and to make sure that large PDF files do not have the option Shrink to Fit selected.