QPDF [1] is a structural editor for PDF files. This description places it in a very specific niche. In its usual output method, it does not edit the content of PDF files – to the extent that editing content is possible, opening a PDF in LibreOffice is generally the easiest way to work. Nor does QPDF import PDFs to different formats – the repositories of major distributions like Debian are full of scripts for that, like pdf2htmlEX and pdf2svg. However, if you need to change how a PDF is put together, QPDF is a toolkit that is both comprehensive and more convenient than the assorted scripts that only perform a single function. In fact, by adding options, you can make an entire series of structural edits with a single command. QPDF is especially handy if you no longer have the file from which a PDF was generated and are therefore unable to make a new one with different settings.

QPDF is available in many distributions. If it is not in your distribution's repository, you can download the source code from the project site and build it with the usual trio of commands: configure, make, and make install. The syntax, too, is simple:

qpdf OPTIONS ORIGINAL-FILE OUTPUT-FILE

The output file is not needed for some options, such as those for information. Commands complete without any confirmation except the return to the prompt.

The original file is kept untouched when the command is run, so any errors will not leave you with a corrupted file. Detailed help is available from the command qpdf --help rather than the man page. QPDF's options are numerous, but the most generally used options can be divided into four main categories: those for general operations, information, page selection, and encryption. In addition, for the adventurous, QPDF can create a file in QDF mode, which will create an output file that can be opened in a text editor.

Options for General Operations

These options determine how QPDF runs, and most can be used alongside other options. They include several unusual features. For example, QPDF's own completion tool can be enabled for either the Bash or Zsh shells with the command

eval $(qpdf --completion-bash)

or

eval $(qpdf --completion-zsh)

If QPDF is not in your path, you will need to give its complete path in order to use completion.

Similarly, if a PDF is protected by a password, in order for QPDF to manipulate it, you will need to give the password with the option --password=PASSWORD. Without the password, even the information options will not function. If, as often happens, the original PDF has two passwords, one for viewing and one for editing, you will have to enter options for both passwords unless they are the same.

If you want a PDF that displays quickly on the web, select --linearization. This display makes such changes as reducing the resolution of images so that they load faster. Documents that are all text will benefit minimally from linearization.

Information Options

QPDF's options for retrieving information about a PDF are useful for troubleshooting or for using the QPDF library in automated test suites (Figure 1). The --check option gives a quick summary of the file structure, encryption, and linearization. The function of other information options is evident from their names (Table 1).

Figure 1: Information options like --show-encryption display detailed information about a file's structure.

Page Selection

QPDF can manipulate the pages shown in the output file. The --pages option must be used after the basic command, and the page range after the original file. Individual page numbers can be separated by commas, or a range of pages by a dash. Individual pages and ranges can be listed together, so that 3,5,11-14 would be a valid listing of pages. Pages are printed in the order that they are listed, so 11-14,3,5 prints pages 11-14 first in the output file. Other values include z-1 to print in reverse order starting from the last page, and r2-r1 prints the last two pages, while r1-r2 prints the last two pages in reverse order.

Output files can also be created that use multiple source PDFs. When using multiple files, place the --pages=PAGES option after the name of each source file, rather than after the basic command. After the basic command, you can add --collate so that the output file begins with the first page or range for the first file in the command, followed by the first page or range for the second file, then the second page or range for the first file, and so on. For example:

qpdf file first.pdf pages=1-4 second.pdf pages=r4-r1 merged.pdf

Still another way to select pages is to define a particular page as either an overlay or an underlay, in effect creating a watermark. Whether you use an overlay or underlay is a matter of choice, usually determined by what you want to be displayed clearly. --overlay or --underlay is added after the basic command, and the first page specified for the first file becomes the overlay or underlay for all the pages specified in the second file. Alternatively, where the overlay or underlay file is applied can be specified by adding --to=PAGES and --from=PAGES after it.