Image optimization lets you balance file size against visual quality. The best balance depends on how you plan to use the file. Nitro PDF Pro uses two techniques to optimize images: downsampling and compression.
Downsampling
Downsampling reduces the resolution of an image. Downsampling lowers the number of dots per inch (DPI) in the image. A lower DPI stores less information, so the PDF file size becomes smaller.
Downsample an image based on the resolution of the device that will display or print the file. For example, a typical computer monitor displays a resolution between 72 DPI and 96 DPI.
To downsample an image:
- Right-click the image and select Properties
- Click the Resolution tab
- Select Downsample Image
- Select a DPI value
Compression
To view these settings, click File > Optimize PDF
Reduced Size (for screen reading)
This setting optimizes a file for on-screen viewing.
- Nitro PDF Pro compresses images using the JPEG compression technique.
- Nitro PDF Pro downsamples images with a resolution above 150 DPI.
Use this setting to create the smallest file size. Use this setting when print quality is less important than file size, for example, when you prepare a PDF for a website.
Reduced Size and Print Ready (for printing)
This setting optimizes a file for standard home or office printers.
- Nitro PDF Pro compresses images using the ZIP compression technique.
- Nitro PDF Pro downsamples images with a resolution above 300 DPI.
Use this setting to keep good print quality on a general-use printer while keeping the file size small. For example, use this setting when you prepare a PDF to email to coworkers.
Custom
The Custom setting lets you apply different settings to different image types in your document, such as color or grayscale images. Available options depend on the image types in your document.
- Downsample (DPI): Sets the DPI for each image. You can keep the original DPI, or choose a lower DPI. A lower DPI reduces print quality and reduces file size.
- Compression: Sets the compression type. Select Automatic to let Nitro PDF Pro choose the best compression type for most images in the document. You can also select a different setting to balance file size against image quality.
- Image Quality: Sets the image quality when you select JPEG compression. Choose the setting that matches how you will use the document. A higher image quality creates a larger file.