How Can I Tell What’s Inside a PDF? Raster, Vector, Text?

So, a PDF is a PDF, right? On the surface, that might seem so, but PDF content, such as graphics and text, can belong to different data types. Depending on how your PDF was created, you will have a vector- or raster-based PDF on your hands. And that can make all the difference in your ability to interact with, measure, and search the content.

Telling vector from raster PDFs may not be possible with a simple visual inspection, as the differences are not evident immediately. So, in this post, we share a few tips & tactics to help you determine what's inside your PDFs.

We’ll look at the most important content in PDFs from an interactive standpoint: graphics and text.

Graphics in a PDF document are embedded as one of two types of data: raster or vector.

Text is an interesting case: it can be an exception to the saying, "If it quacks like a duck..." Text characters in a PDF might look like text but not be "true" text elements. If text is defined by raster dots or vector line segments only, the text isn’t machine-readable text. This type of "text" is essentially an image, and you can’t search, highlight, or edit characters in an image. For text to be true searchable text, it needs an invisible layer of character encodings. (Most PDFs use the standard UTF-8.)

Ask yourself whether you’ve ever tried to do something with a PDF, unsuccessfully. Perhaps you couldn’t do any of the following:

The reason is probably that you had a raster PDF on your hands.

This is somewhat of a simplification, but most PDFs are vector files; you can also save PDFs as raster files. And each graphics type is built for specific purposes. Whether to use vector- or raster-based PDFs depends on knowing which is best for the job you want to do.

You can easily figure out whether the content of your PDF is vector or raster using a few tricks or tools:

Your best bet for most situations is to zoom in to a detailed part of a PDF, to greater than 800% magnification. Vector PDF file content will look clear and smooth at any magnification while raster PDF content will become blurrier and more pixelated the more it’s zoomed.

If you can’t display text using a file menu option (such as Show All Text), then the text in the PDF is a raster image or vector lines without character encodings. Therefore, it is not “real” machine-readable text. Raster “text” is an image in which you cannot search text, select text, or edit text. The same goes for vector lines used to create the appearance of characters but without encodings.

PDFs created from character-based programs (e.g., Word and Excel) almost always create PDFs that contain real text.

You can run optical character recognition (OCR) on a PDF if you need to convert raster and vector images into searchable text. OCR interprets the images on a scanned PDF and creates an invisible text layer on top. This layer allows you to access real text that would otherwise be unavailable for manipulation in an image.

Simple methods like zooming in are great if your PDF contains a huge image, such as a drawing of a building, and you just want to quickly verify its contents.

But what if you want to look at different elements on an individual level or across many pages?

Some tools can help shed light on what’s image content quickly, so you can work efficiently.

Adobe Acrobat Pro lets you audit space usage to determine whether a PDF contains a lot of image content.

Choose File>Save As Other>Optimized PDF, or Go to Tools>Optimize PDF, and then click Advanced Optimization. The PDF Optimizer dialog box opens.

Click the Audit Space Usage button at the top of the dialog box.

You can also use a tool like CosEdit to examine the low-level content of a PDF (Cos Objects, which represent components in a PDF file). PDF CosEdit is a stand-alone, low-level PDF editor. It allows you to create, browse, and edit PDF and other COS-based documents at the object level.

Using a low-level PDF editor provides insight into what’s inside a PDF.

Downsampling is the process of changing the resolution of an image, usually done during compression to reduce the PDF file size. Downsampling makes raster content low quality and thus its presence obvious.

You can downsample a PDF programmatically. Use the Apryse SDK and Class Optimizer. Specify a very low value for DPI (like 1) for ImageSettings and MonoImageSettings.

We hope that this blog has shed light on the types of data contained in PDFs and how to figure out what your PDF contains – raster, vector, or text.

If you’d like to deepen your understanding of PDF rendering and PDF viewer libraries, check out our articles: