The Portable Document Format (PDF) offers multiple compression options to achieve the optimal balance between file size and quality. The choice of compression depends on various factors, including the type of content within the PDF and the desired output quality. In this blog plot, I will share the knowledge about these factors.
What is the difference between lossy and lossless compression?
Lossless Compression: Preserves the original data perfectly. When uncompressed, the data remains identical to its original form.
Suitable for: Text, file offset locations in a PDF, Vector Graphics, and other content requiring precision.
Lossy Compression: Some data is discarded to achieve smaller file sizes, which might reduce quality but often goes unnoticed in certain scenarios.
Suitable for: Colour images where slight quality reduction is permissible.
Which is the best option to compress a PDF?
The ideal compression depends on your document’s content and your objectives. There is generally a tradeoff between file size and image quality. The PDF compressions algorithms represent different best options depending on which is most important to you. Read the question “Which is the best compression for images and text” to know it.
How do I compress a PDF without losing quality?
If preserving detail is crucial, especially in text or vector graphics, opt for lossless compressions like CCIT, Flate, JBIG2, LZW, RLE, ZIP. For colour images lossy algorithms (JPEG(DCT), JPEG2000(JPX) ) would not be noticeable and give us smaller files.
Which type of image compression has the best quality?
It depends on the type of image. There are different compression algorithms for colour and Black and white images. You can read about the options in the question “Which is the best compression for images and text?”.
What types of content are found in a PDF document?
PDFs are versatile and can encapsulate various types of content.
1. Text and Vector Graphics: These are primarily stored in binary streams. Due to their precise nature, they require lossless compression to retain their original quality and accuracy.
2. Images: Images in PDFs are kept as distinct XObjects. Depending on the need for quality versus file size, the pixel data of an image can be compressed using either lossless or lossy formats. It has additional attributes like
ColorSpace: This defines the image’s color spectrum. For precision, it’s always subjected to lossless compression.
Mask: This determines image transparency, and should be stored using lossless compression for true representation.
3. Inherent PDF Objects: These are the foundational objects that constitute a PDF document. These objects must always be compressed using lossless algorithms to ensure the integrity and accuracy of the document’s data.
Which is the best compression for images and text?
CCITT: is best for black and white images which it is designed to compress very efficient. There are different groups of CCITT, with Group 4 being the most common in PDFs. It provides efficient compression for monochrome images. You can read our blog on What is CCITT compression.
Flate: is used for text and mixed content documents. It’s a versatile lossless compression that works well with text and image data. It’s one of the primary methods for compressing content in PDFs.
JBIG2: is used for Bi-level (black and white) images. It offer significantly better compression ratios than CCITT Group 4, especially for scanned text pages. It can be lossless or lossy.
LZW: is used for ext and moderately detailed images. It is a lossless method, historically used in GIFs and TIFFs. You can read about it more in our LZW compression blog.
RLE: is used for data with large sequences of repeated bytes, like monochromatic images. It is a simple form of lossless compression where runs of data are stored as a single data value and count.
ZIP: It is a general-purpose compression for text and images. ZIP in PDF compression context is essentially the Flate method. It’s lossless and provides decent compression ratios.
JPEG (DCT): It is used for full-color photographs. It’s a lossy compression method that works by transforming the image data into frequency space. It discards frequencies which are less noticeable to the human eye, leading to significant reductions in file size. However, it can introduce artifacts. You can read in our blog post on JPEG
JPEG2000 (JPX): It is used for high-quality images and photographs. It can offer both lossless and lossy compression. It provides better compression ratios and fewer artifacts than traditional JPEG. You can read about it in our blog on JPEG2000.
Can I use multiple compression methods in a single PDF?
PDFs can use different compressions for different elements. For example, you could use lossless compression for text and vector graphics and lossy compression for images within the same document.
Does encrypting a PDF impact its compression?
Encryption and compression are distinct processes. Once a PDF is encrypted, additional compression might not be effective since encrypted data doesn’t compress well. It’s often recommended to compress first, then encrypt.
Is there any impact on the speed of rendering with different compressions?
Yes, more aggressive compressions might reduce file size, but they can also increase the time it takes to decompress and render the content. But smaller files are quicker to download.
Want to explore more. Dive into our blog top 9 pdf questions with answers for developers to enhance your knowledge.
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