This is a three-part process:
- Print proofing. Checking that the document will be able to print correctly, print proofing.
- PDF creation. Creating a print-ready PDF.
- PDF quality control. Checking the quality of the PDF.
The preflight verification is a technical check on the content of the layout document:
- Are font files present and in the correct format?
- Are all image files present and of the correct resolution and color space?
- Is the page size correct?
- Is the color handling correct (is the document in the correct color space)?
You can not be sure that the PDF document is correct until you run a preflight verifier on it (and not even then – it still has to go through a RIP, Raster Images Processor).
Part 2: PDF creation
The page layout software is used to create a print-ready PDF document which is sent to the print shop.
Settings for the PDF creation has to be done according to what kind of print process is used. (For Adobe software the settings are sometimes included in a so called joboptions-file supplied by the print shop.)
The CMYK conversion, necessary for professional printing, is done at the same time (except for PDF/X-3). This step needs an ICC-profile, supplied by the print shop.
Part 3: PDF quality control
A PDF preflight program can check many things inside the PDF, like image resolution, font embedding etc. It can also check that the PDF is of the right version.
For very reliable color reproduction the print shop can produce a paper proof before printing all copies.
A preflight verifier exists in Scribus. It checks that all content in the document is visible and that images are present and of the correct resolution.
Part 1: Print proofing
Examining the previously mentioned Adobe's list of the top 10 problems with PDF:s, we can see that different methods (and software) have to be used for each problem:
- The resolution of images is too low. This is checked with the Scribus preflight verifier.
- Fonts are not embedded in the PDF. In Scribus, make sure you have embedded all available fonts (File > Export > Save as PDF > Fonts tab). Also check that all fonts are embeddable (see Scribus Setup).
- The wrong color space is used. In Scribus, check File > Document Setup > Color Management. Is the correct ICC profiles assigned (see Step 3a. Configure Scribus in the Color Management tutorial)?
- The information about trim or bleed are incorrect. Make sure you have assigned (the correct) bleed values in the File > Document Setup window (or when creating the Scribus document). Also remember to check Use Document Bleeds in the File > Export > Save to PDF > Pre-press tab.
- There is an inconsistency with the native file. The original page, as viewed in the layout application looks different from the PDF. Hairlines might be different or gradients have changed. This can only be checked after the PDF document creation. A printout can possibly expose some problems. Otherwise, order a printed proof from the printer.
- A spot color is misnamed or it is accidentally converted to a process color. If you do want spot colors (which you most often don't want when doing normal four color CMYK printing), check Convert spot colors to process colors in the File > Export > Save to PDF > Colors tab.
- Images are compressed too much. This causes a quality loss and in some cases artifacts appear inside or around the images. In the File > Export > Save to PDF > General tab, set the Compression Method to Automatic and the Compression Quality to Maximum.
- The page size is incorrect. Make sure you have assigned the correct width and height in the File > Document Setup window (or when creating the Scribus document). Also remember to use the correct values for bleed and the correct printer marks in the File > Export > Save to PDF > Pre-press tab.
- There are problems with transparent objects. Try to avoid using transparency if at all possible.
- ICC profiles are missing or incorrect. Examine images in Scribus and check that they are tagged with the correct ICC profiles. Use the Image tab in the Properties window.
2. PDF creation
PDF creation is fully supported in Scribus.
3. PDF quality control
Libre PDF preflight software is not available except for a somewhat limited tool written in Ruby.
It's also possible to check that the PDF document is of the right version using PDFTK by entering the following on the command line:
pdftk mydoc.pdf dump_data
The document is sent to the print shop as a PDF document. There are several versions of the PDF format. Here are some common ones:
- PDF 1.4 and PDF 1.5 handle transparency.
- PDF/X-1a specifies that all colors are CMYK or spot colors.
- PDF/X-3 tags all images with the right ICC profile and leaves the CMYK conversion to the RIP (Raster Images Processor) at the print office.
- PDF/X-1a and PDF/X-3 are based on PDF 1.3 which does not handle transparency.
Ask the print shop. What is most common also seems to differ in different countries. If you don't know, PDF 1.4 is a good choice.