Object streams are a very useful feature added to the PDF specification which introduces a new type of PDF object. Until they arrived, PDF objects consisted of a binary part (which could be compressed) and a text header (which was not). If you open a PDF file in a text editor, you would see something like this.
TEXT containing information about the PDF object
15 0 obj<</Type/XObject/Subtype/Image/Width 153/Height 66/ColorSpace/DeviceRGB/BitsPerComponent 8/Interpolate false/SMask 16 0 R/Filter/FlateDecode/Length 3609>>
This is still allowed and you can continue to do this – the PDF file format has always been very good on backward compatibility. But what object Streams allow you to do is to put lots of PDF objects together inside a single binary stream. The binary stream still has a text header, telling the PDF parser how to find and extract the PDF objects, but all the PDF objects themselves can be compressed. This makes the PDF smaller, potentially more secure and possibly faster to load.
The only minor downside is that developers like me can no longer open the PDF files in a text editor and find the objects – they are now hidden away inside compressed binary data! But there are lots of tools to allow us to see inside the PDF, some of which we have highlighted in Useful PDF tools – pdfedit.
This feature was introduced in PDF version 1.5, so you will need a tool which can support Object streams. It is the major reason many PDF files do not open in Sun’s PDF-Renderer, for example. But it is now supported by most mainstream PDF tools.
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This post is part of our “Understanding the PDF File Format” series. In each article, we aim to take a specific PDF feature and explain it in simple terms. If you wish to learn more about PDF, we have 13 years worth of PDF knowledge and tips, so click here to visit our series index!
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