Converting PNG to PDF on Ubuntu: A Comprehensive Guide

Converting images from PNG (Portable Network Graphics) format to PDF (Portable Document Format) is a common task for various reasons, including archiving, sharing, and printing. If you're using Ubuntu, you might want to perform this conversion using only the default packages, without installing any extra software. This article explores different methods to convert PNG to PDF on Ubuntu using the command line, focusing on the convert command and other built-in tools.

Why Convert PNG to PDF?

Before diving into the how-to, let's understand why you might need to convert a PNG to a PDF:

• Document Archiving: PDFs are excellent for archiving documents because they preserve formatting and are widely supported.
• Easy Sharing: PDFs are universally readable across different operating systems and devices, making them ideal for sharing.
• Printing: While PNGs can be printed, PDFs offer more control over print settings and layout.
• Combining Multiple Images: PDFs can contain multiple images within a single document, which is useful for creating portfolios or presentations.

Using the convert Command (ImageMagick)

The most straightforward way to convert PNG to PDF in Ubuntu is by using the convert command, which is part of the ImageMagick suite. ImageMagick is a powerful image manipulation tool that is often included by default in many Ubuntu installations.

Single File Conversion: To convert a single PNG file to a PDF, open your terminal and use the following command:

convert image.png image.pdf

Replace image.png with the actual name of your PNG file. This command creates a PDF file named image.pdf in the same directory.

Batch Conversion: If you have multiple PNG files to convert, you can use a loop in the terminal:

for file in *.png; do convert "${file}" "${file%.png}.pdf"; done

This command iterates through all PNG files in the current directory and converts each one to a PDF with the same name. For example, photo1.png will be converted to photo1.pdf.

Merging Multiple PNGs into a Single PDF: To combine multiple PNG images into a single PDF file, use the following command:

convert *.png output.pdf

This command merges all PNG files in the current directory into a single PDF file named output.pdf.

Troubleshooting: Security Policy Errors

Sometimes, you might encounter an error message like "not authorized h1.pdf' @ error/constitute.c/WriteImage/1037" or "attempt to perform an operation not allowed by the security policy PDF". This is due to security restrictions in ImageMagick's default configuration. To resolve this, you need to modify the policy.xml` file.

1. Locate the policy.xml file: The location may vary based on your ImageMagick version, but it is often found in /etc/ImageMagick-6/policy.xml or /etc/ImageMagick-7/policy.xml.

2. Edit the file: Open the file with a text editor using sudo, for example:

   sudo nano /etc/ImageMagick-6/policy.xml
   

3. Comment out or modify the PDF policy: Look for the section related to PDF and comment it out by wrapping it in <!-- -->, or adjust the rights. For example:

   <!-- <policy domain="coder" rights="none" pattern="PDF" /> -->
   

   Or change the rights:

   <policy domain="coder" rights="read|write" pattern="PDF" />
   

4. Save the file: After making the changes, save the file and try the convert command again. Disclaimer: Modifying the policy.xml can introduce security risks if not done carefully. Only make necessary changes.

Using Image Viewer

Another option is to use the default Image Viewer in Ubuntu.

1. Open the PNG image with Image Viewer
2. Click on File then Print
3. Select Print to File and choose PDF as the output format.

While this method gives you a graphical user interface, it is less suitable for batch processing.

Additional Tips and Considerations

• File Order: When merging multiple PNG files into a single PDF, the order in which the files are listed matters. Use the echo *.png command to preview the order.
• Dependencies: The convert command relies on the ImageMagick package. If it's not installed by default, you can install it using sudo apt install imagemagick.
• Alternatives: While the focus is on default packages, other tools like img2pdf (img2pdf on GitHub) can also be used for PNG to PDF conversion, offering additional features and optimization options.

Conclusion

Converting PNG images to PDF format in Ubuntu using default packages is a straightforward process, thanks to the convert command and Image Viewer. Whether you need to convert single files, batch process multiple images, or merge them into a single PDF, these methods provide efficient solutions. Remember to handle security policies with care and consider additional tools for advanced features.