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How are you copying a section of the file? When you say you "copy" from the PDFs, does that mean that you are not inserting the entire original PDF file? If you insert the entire file (or simply drag it onto the page), the quality will be preserved. However, most ways of copying a portion of a PDF involve making a low-resolution (rasterized) version of the copied portion.
If you only want a small section of the PDF, one way around this problem is to open the original PDF in Preview, then use the Select tool to select the area you want, and then choose Crop from the Tools menu, and do a Save As of the resulting file. This procedure will preserve the quality of the PDF, and just create a new PDF file. You should be able to drag this new file into Pages without a loss of quality.
The better way, however, may be to simply use Pages' Mask with Shape functionality. Insert (or drag) the original PDF into Pages. Then create a new shape on top of the PDF image, sizing and placing it where you want the visible section to be. Then, select both the shape and the PDF, and from the Format menu choose Mask with Shape. The part of the PDF outside of the shape will be "masked" or hidden, and the quality should be preserved. Also, an advantage of this method is that you can go back and edit the mask to show more or less of the PDF (and even have non-rectangular masks, if you like).