PDF on Linux tips

[Jan. 2020]

Occasionally I find that I need to massage PDF files in some way. Since I use Ubuntu as my desktop operating system, I need to find software and methods that will work with Linux (and are free!). There is actually a large selection of Linux tools for this, both command line and GUI type (although the command line tools tend to be the most useful). Since some tools work better than others, or work better on particular PDF types, I plan to keep a record here of the tools I find most useful. As the need arises, I will include entries on these topics:

• Bursting - converting a multi-page PDF to single PDF pages

• Compression - reducing PDF file size

• Enhancement - improving contrast, removing speckles, etc.

• Image to PDF

• Merge - Multiple PDF files to a multi-page PDF

• OCR

• Password removal

• PDF to Image

• Splitting - converting 2-up pages to 1-up pages

• Watermarks - removing and adding

Bursting

Compression

There are a variety of Linux tools available to reduce the file size of a pdf:

• ps2pdf :

  • [command line] ps2pdf LARGE.pdf SMALL.pdf

  • or

  • [command line] pdf2ps large.pdf - | ps2pdf - small.pdf

• qpdf : [command line] qpdf --linearize input.pdf output.pdf

• ghostscript :

  • [command line] gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=out.pdf in.pdf

  • see also : [command line] gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET

• ImageMagick : [command line] convert -density 200 -compress jpeg -quality 20 test.pdf test2.pdf

Enhancement

There are a variety of options for enhancement:

• gIMP image editor

• Libreoffice Draw

• Command line

gIMP image editor

This is really an option of last resort as it is very time consuming, but it is a very powerful tool for major issues. Explaining the full scope of using gIMP would require a book, but the general process is to convert the PDF page (or pages) that need repair to images, and then editing the images. Depending upon the level of effort put in, this can include straightening out skewed pages, removing scribbles, blots, etc., filling in obvious gaps in drawings and images, improving contrast, and so on. Once the image has been edited, it is converted back into a PDF and re-integrated with the full document.

LibreOffice Draw

PDF files can be opened in Libreoffice Draw, with mixed results. It is worth trying, as in some cases it works very well. However, with some PDF files the result is scrambled pages, missing images, etc.

Command Line

[to be added later]

Image to PDF

https://askubuntu.com/questions/493584/convert-images-to-pdf

Command line : convert "*.{png,jpeg}" -quality 100 outfile.pdf

Merge

See this link : https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf#11280219

pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf

OCR

It's very common to find PDF files which are scanned images of pages; since the text is an image it cannot be searched or cut&pasted. Fortunately there is OCR software which can create an "image over text" copy of the original PDF file. I have found three useful tools for this purpose. Note that all of these tools require the tesseract OCR engine - this can be installed form the Ubuntu software repository if you have not already done so. Here are some OCR options (see also this page):

• pdfocr : https://raw.githubusercontent.com/gkovacs/pdfocr/master/pdfocr.rb

  • More explanation here : https://ubuntuforums.org/showthread.php?t=1456756

  • This is a script which you run from the command line; I have found this produces relatively fast and clean copies.

    • Change one line in the script from

    • use_tesseract = false

    • to

    • use_tesseract = true

    • Run the script from the command line as:

    • pdfocr -i input.pdf -o output.pdf

• gscan2pdf : This is GUI based. It works, but I found it slow, and in some cases it produces PDFs with washed out or missing images.

  • Install this from the Ubuntu software repository

• pdfsandwich : http://www.tobias-elze.de/pdfsandwich/

  • This works, but also sometimes produces results with poor image pages.

  • This suggestion to use the -nopreproc option helped me get better results : https://www.onetransistor.eu/2015/12/ocr-searchable-pdf-linux.html

    • e.g [command line] pdfsandwich -nopreproc in.pdf

      • (results in in_ocr.pdf generated)

Password Removal

Occasionally I will run across a PDF file which permits reading, but is otherwise password protected (why!? annoying and pointless!). In such cases I have found that simply opening up the PDF in a PDF reader and then printing it to a file creates a copy with no password. Printing to a PDF file is built into Ubuntu Linux printing.

PDF to Image

For a single page PDF use GIMP. For a scanned PDF this also provides an opportunity to clean up or enhance the image before exporting to an image file.

For a multi-page PDF :

pdftoppm {input.pdf} {output.file} -png

See this link : https://www.cyberciti.biz/faq/how-to-convert-pdf-to-image-on-linux-command-line/

For a page range :

convert -density 150 input.pdf[119-121] -scene 10 -quality 100 output-%d.png

In the example above, pages 199-121 will be output as :

output-10.png
output-11.png
output-12.png

Watermarks