How to Flatten a PDF File in Linux

1. Introduction

PDF, or Portable Document Format, is an Adobe-created file format for cross-platform document sharing with formatting preservation. Flattening a PDF to an image involves consolidating layers and interactive elements for document simplification and security. Thus, it’s commonly done for finalized documents where further edits are unnecessary.

In general, flattening a PDF to an image refers to merging or combining multiple layers within the PDF into a single layer. In other words, flattening simplifies the structure by merging these layers, resulting in a single-layer image in the PDF file. This process is commonly used for finalizing and distributing files that don’t require text selection.

In this tutorial, we’ll learn several ways to flatten a PDF image in Linux.

First, we’ll explore the ImageMagick convert tool and Poppler utils to flatten a PDF. Next, we’ll discuss a PostScript approach that consists of the pdf2ps and Ghostscript tools. Lastly, we’ll look into PDF Toolkit (PDFtk).

2. Using ImageMagick

To begin with, we can use the ImageMagick convert tool to flatten a PDF file.

ImageMagick is an open-source software suite for raster image file tasks. To flatten a .pdf file, we can use its convert tool.

convert is a command-line utility that plays a central role in executing a variety of image operations. Additionally, the convert tool is well-suited for batch processing and automation due to its extensive options. As a result, it’s widely used for diverse image processing requirements in fields such as web development and graphics design.

To employ convert, first, we install ImageMagick. For that, we use the APT package manager:

$ sudo apt install imagemagick

Once the installation is complete, we check its version to verify its installation:

$ magick -version

If the installation is successful, we see its version number.

After that, we can open a new terminal and use the ImageMagick convert tool to flatten a PDF file:

$ convert -density 300 original.pdf imageMagic_flatten.pdf

The above command takes original.pdf, flattens it with a resolution of 300 DPI, and saves it as imageMagic_flatten.pdf. To elaborate, this command merges layers and interactive elements in original.pdf for simplification.

In essence, ImageMagick and its convert tool provide a robust solution for professionals seeking efficient and flexible approaches to address various image-related tasks.

3. Using Poppler Utilities

Similarly, we can use the Poppler library to flatten a PDF file. The Poppler utilities are command-line tools associated with the Poppler library, designed for manipulating PDF files.

There are several Poppler tools available for automation, improving interoperability, and facilitating tasks, including pdftocairo, pdftoppm, and pdfimages.

Here, we use the pdftocairo tool to flatten a PDF document:

$ pdftocairo -pdf original.pdf pdftocario_flattened.pdf

The above command uses the pdftocairo command to convert original.pdf to another file named pdftocairo_flattened.pdf. The -pdf option specifies the output format as PDF, implying a potential flattening process for simplifying the document.

4. Using pdf2ps

Another approach to flatten a file is to use the pdf2ps tool. pdf2ps is a command-line tool that efficiently converts PDF files to PostScript format.

Since pdf2pdf is available by default in Ubuntu, there is no need to install it. For other Linux distributions, we can use the included package manager or the official Poppler tools package.

Let’s see how we can use pdf2pdf to flatten a PDF file:

$ pdf2ps original.pdf - | ps2pdf - pdf2pdf_flattened.pdf

The above command involves a two-step process. Firstly, the pdf2ps tool converts the input original.pdf file to a PostScript format, with the output directed to the standard output. Secondly, the PostScript output is piped to the ps2pdf tool, converting it back to the PDF format, and saving it as pdf2pdf_flattened.pdf.

At this point, pdf2pdf_flattened.pdf should contain the original document in flattened form.

5. Using Ghostscript

Alternatively, we can use Ghostscript to make any PDF file flattened.

Ghostscript interprets and renders PostScript or PDF files, facilitating conversion, printing, and PDF manipulation. Furthermore, its open-source nature and cross-platform compatibility contribute to its flexibility and widespread use.

As Ghostscript comes pre-installed in Ubuntu by default, there is no requirement to install it separately. Still, we can do so via an official package or from its sources.

Now, let’s execute the gs command to make a PDF file flat:

$ gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite -dPreserveAnnots=false \
     -sOutputFile=ghostscript_flatttened.pdf original.pdf

The above gs command initiates the Ghostscript interpreter to process a PDF file. The command ensures a safer mode of execution with restricted access and directs gs to exit after processing.

Next, the command specifies the output file as ghostscript_flatttened.pdf, generated from the input file, original.pdf.

Now, let’s view the details of the flags used with the gs command:

• gs invokes Ghostscript
• -dSAFER sets the interpreter into safer mode
• -dBATCH exits the interpreter after processing all input files
• -dNOPAUSE disables the prompt and pauses at the end of each page
• -dNOCACHE disables the caching of objects
• -sDEVICE=pdfwrite specifies the output device as pdfwrite, indicating Ghostscript to produce a PDF file
• -dPreserveAnnots=false doesn’t preserve annotations in the output file
• -sOutputFile=ghostscript_flatttened.pdf specifies the name of the output file

So, in summary, the command uses gs to process the input file and save the result as a new file called ghostscript_flattened.pdf.

6. Using PDF Toolkit (PDFtk)

Another alternative method to flatten a file is to use the PDFtk. PDFtk is a versatile tool available on various operating systems, enabling users to manipulate PDF documents.

Additionally, PDFtk facilitates the flattening of PDF forms for a more streamlined presentation. Its command-line interface makes it suitable for batch processing and automation, addressing diverse needs in PDF document management.

PDFtk isn’t always included in the default software set on Linux, so we might need to install it manually. To kick off the installation, we use the apt command:

$ sudo apt install pdftk

Once the installation is complete, we can execute the pdftk command to flatten a .pdf document:

$ pdftk original.pdf output pdftk_flattened.pdf flatten

In the above code, we use pdftk to flatten original.pdf. The resulting document is saved as pdftk_flattened.pdf.

7. Conclusion

In this article, we learned several ways to flatten a PDF image in Linux.

Initially, we discussed flattening a PDF file using conversion and manipulation tools, such as ImageMagick and Poppler. Subsequently, we looked at the PostScript techniques, encompassing tools such as pdf2ps and Ghostscript. Finally, we explored PDF Toolkit for flattening a PDF file.