Portable Document Format (PDF) is a versatile file format created by Adobe Systems which is totally independent of hardware or software and works on any platform. PDF is an open format which is a container for various vector objects, fonts, raster images, text, etc.
Unfortunately PDF files created by default by most of the PDF creation/conversion software are not optimized. Therefore some tweaks must be performed to make a smaller PDF file. Several third party software such as PDF Shrink and PDF Compress exist which use sophisticated algorithms to optimize and export a smaller PDF file. But this software comes with a heavy price tag. Hence if the user is under a strict budget, several alternatives exist likewise.
If the user works in a Mac OS platform, he/she can use the color sync tweak by going to file>print>color sync>quartz filter and then exporting it as PDF. This optimizes the PDF and compresses the objects and the color table of the PDF document, hence resulting in a smaller PDF file.
Another general method to optimize PDF documents is to down sample the 2D-vector images or objects, hence reducing file size. If the document consists of raster images, they can be re-imported after converting it to JPEG format. Coupled with down sampling, it results in a lot smaller PDF file, which can be about 20-30% lesser than the original file size. Reducing the image file size to the frame size of the PDF document can result in significantly smaller PDF files too. The down sampling is done by default if the PDF creation/conversion software has a compress option.
PDF also allows itself to be compressed by several popular compression algorithms such as LZW, RLE, JPG, JBIG2, etc. These algorithms not only compress images but also text (lossless in nature). Print quality must be wisely chosen, as it affects the file size too. The recommended print quality for normal printing is 300 dpi and that for commercial printing is 1200 dpi. A lesser print quality results in a smaller PDF file size.
Lastly a large PDF file can also be split into several smaller PDF files using softwares such as PDF Split. These files might not be optimized but can be useful during transfer of files using smaller capacity media such as floppy disk or ZIP drives.