PDF files are portable document format files originally created by Adobe. PDF files look the same on every computer, unlike Web pages. By default, PDF files preserve as much image quality as possible. For example, if you convert a high-resolution image to a PDF file, that PDF file remains very large. This is intended so that no quality is lost; if you go to print the PDF, it will have the same quality level as the original image. However, often the exact detail of the PDF doesn't matter, but the file size does.
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Open a Terminal by clicking "Applications," "Accessories" or "System Tools" and "Terminal."
Navigate to the folder containing the PDF file with the "cd" command. For example, navigate to the default Documents directory by typing "cd Documents" into the Terminal window and pressing "Enter."
Type "gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=setting -sOutputFile=output.pdf input.pdf" into the Terminal window, replacing "input.pdf" with the name of your PDF file and "setting" with a desired quality level, and then press "Enter."
Quality level settings are "/screen," the lowest resolution and lowest file size, but fine for viewing on a screen; "/ebook," a mid-point in resolution and file size; "/printer" and "/prepress," high-quality settings used for printing PDFs.