The pdf format takes information from a page created by a graphics program such as Microsoft Word or Adobe InDesign and converts it to a common format that can be read on a variety of devices by the free program Adobe Reader and by the paid program Adobe Acrobat Pro. Adobe Acrobat Pro allows editing and notating the pdf.
Pdfs are good for storing text and images. When including text, the pdf conversion embeds part of the font information so that the original look of the page can be maintained without having to install those specific fonts on the device. Images are usually converted from their original format to jpeg format, with varying degrees of compression.
Graphic designers like pdfs because it is much easier to send a pdf to a printer than to send a properly packaged file with all the images and all the fonts, and if the pdf looks good when you send it, there are fewer chances of printing problems because everything is locked in place.
In Adobe InDesign, there are 6 types of pdfs listed:
High Quality PDF — Intended for printing to laser and inkjet printers. High enough resolution that the images will look good when printed in this way. "This preset uses PDF 1.4, downsamples color and grayscale images to 300 ppi and monochrome images to 1200 ppi, embeds subsets of all fonts, leaves color unchanged, and does not flatten transparency (for file types capable of transparency)."1
PDF/X — PDF-X formats are for sending to professional printers, and do not allow the embedding of interactive elements such as forms or video. Specifics:
Press Quality — For sending to professional printers. "Creates PDF files for high-quality print production (for example, for digital printing or for separations to an imagesetter or platesetter), but does not create files that are PDF/X-compliant. In this case, the quality of the content is the highest consideration. The objective is to maintain all the information in a PDF file that a commercial printer or print service provider needs in order to print the document correctly. This set of options uses PDF 1.4, converts colors to CMYK, downsamples color and grayscale images to 300 ppi and monochrome images to 1200 ppi, embeds subsets of all fonts, and preserves transparency (for file types capable of transparency). These PDF files can be opened in Acrobat 5.0 and Acrobat Reader 5.0 and later." 1
Smallest File Size — This setting is intended for proof files sent over the Internet to be viewed on a computer screen. It uses a very high rate of compression on images, downsamples them to 72ppi, is not intended for printing, and should never be sent to a professional printer as anything other than a proof.
WARNING — While most of these pdf preset downsample images to 300ppi, PDF PRESETS DO NOT CHECK TO SEE IF YOUR IMAGES ARE 300ppi OR LARGER TO BEGIN WITH. If your images are less than 300ppi at 100% (the standard for professional printing) before creating the pdf, they will not magically be made 300ppi at 100% by conversion to pdf, and will print soft or pixillated on press. The printer is not responsible for this. Always check your image resolution before conversion to pdf.
There are additional options when making pdfs other than those directly available from these presets. Some things to be aware of:
File>Adobe PDF Presets>Define. Select the profile you want to base yours on, and click New. Make changes to the presets, create a name for your preset, and click Okay. The new preset name will appear in the list. If you are basing your preset on another, it's good to include the original name as part of the name.
1 Source: Illustrator / Creating Adobe PDF files