Rasterization is the process of converting resolution-independent PostScript outlines or vector graphics data into fixed-resolution pixels. Printers may need to rasterize files to combine fonts, logos, and photos into a single image of pixels. In most cases this is process is transparent to the user -- except for when there is a long delay between pressing the Print button and actually retrieving a print from the tray. That time represents the rasterization time, which is shortened by faster processors and increased RAM (random access memory).
On occasion, it may be necessary to rasterize something when preparing a file. Though can in many cases increase the file size, in some situations it can produce smaller, easier to print files. For example, Adobe InDesign supports inclusion of PDF files as pictures within a document. The PDFs may be large multiple pages or contain many photos. When placed within an InDesign document, the printer must rasterize that larger PDF even if it appears at a smaller size of a few inches.
A PDF can be opened within Adobe Photoshop. Doing so presents the user with a dialog box asking for the resolution or PPI (pixels per inch), color space (RGB, CMYK, Grayscale), and dimensions. Indicate the desired width, specify 300 PPI, and choose the appropriate color space. (Refer to CMYK and RGB glossary definitions.)
It is possible to export a PDF from an Adobe InDesign or Illustrator document that contains a combination vector and rasterized images (photos). When this is done, the PDF retains resolution-independent vector data and crisp text along with the images at the specified resolution.
Saving a Photoshop document as a PDF with editable text layers also preserves sharp vector information when viewed in Adobe Acrobat (Reader). The PDF can be reopened within Photoshop for text editing. However, when using Photoshop to open PDFs created by other applications (like InDesign, Illustrator, QuarkXPress, or Word), text and vector data are rasterized at the resolution specified in the open dialog box. This means that (particularly small) text has the potential to degrade when enlarged, as the inset example shows. (Top: Screen capture of editable 18-point text layer in Photoshop when zoomed in 300%; Bottom: Screen capture of same document saved as PDF as viewed in Adobe Acrobat.)
To preserve the integrity of vector graphics and the ability to edit text, designers generally assemble elements in a page layout application like Adobe InDesign. Photographs can be inserted at 300 PPI or higher resolution. From here, a PDF can be created with lower resolution for the Web or client proofing, and another at full resolution for final printing.