When the layout’s “done,” the PDF has been exported, and the file name reads “final,” that is when preflighting should be done. A file can be fully “designed” but the page size is wrong, bleed is missing, images are too low-res, text is in the danger zone or fonts did not embed/export. Preflighting is the technical step between you (or the working file) and your printer.
Start by opening the exported PDF in your PDF application and review the page size information. Verify page dimensions are set to the finished dimensions plus bleed (if bleed has been added). Check the number of pages and their order for items like a folded brochure or a document that may be printed double sided. Remember, if the layout has been exported using the wrong PDF preset, extra or missing pages might be included, or the page orientation or dimensions can be inherited from an old layout.
Next, review the bleed area for all elements. The bleed area is reserved for background colors, photos, shapes, images and other elements that were placed flush with the finished edge of the document. Crop marks, if required, should reside outside the finished size, and not overlap with the layout or design elements. Review items close to a trim edge, such as a thin frame. Crop marks might create a slight variation in the thickness of the frame.
Now check the resolution of all images in your PDF file and the color space used by all elements. The resolution of every element in the layout should be considered at its size, so a high-resolution image placed smaller will have a much higher resolution than the same file placed full-page. Review any RGB images if the output is to be CMYK and check that image is not in the over-rich ink range. Be aware that bright screen colors can change color when converted, so images, logos and text that rely on accurate colors should be reviewed after conversion to the intended color space.
As an exercise for preflight, open two exported versions of a PDF file of the same design. In one of the two files, purposely leave out the bleed or place an element in the crop area or use an incorrect setting when exporting one image in the file. Review the properties of the page size, bleed and crop area of both PDFs. Review each image and zoom to compare the detail of the images between the two exported files. By identifying errors planted in one file, it becomes easier to inspect each PDF file exported before sending it to the printer.
Also inspect the font information of the exported PDF file, as font substitution can occur during export. Zoom into any text with a small point size, text using light lines or reversed type on a dark background. Review that the fonts have been embedded or subsetted (if required), and identify any font substitution or missing characters or images. Reviewing the output of any fonts placed in overprint areas is useful, especially for text in a dark color, colored elements on top of black or white elements in overprint mode.
Finally, think of the design file as a production handoff. Identify only one as the final file and ensure that any other working files are not included. If required, write a production note, describing the size, color process, paper, quantities and any special finish or trimming options. Before exporting or sending a print-ready PDF to a printer, ask yourself these questions, at full scale: is the correct page size, bleed and crop included, are all elements acceptable for printing or can all elements still be used as they were placed, and is it possible for the printer to know without guessing what the final file is for?