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Compression and downsampling do different jobs
A PDF can contain images with more pixels per inch (ppi) than are useful for its intended screen or print output. Downsampling reduces an image’s pixel dimensions by combining pixels. This can reduce file size, but it also removes detail that may matter at the final viewing or printing scale.
Compression is the method used to encode image data. A compression method can represent the same image data more compactly without discarding it, or reduce size by discarding information. In Adobe Acrobat’s PDF Optimizer, image controls are grouped for color, grayscale, and monochrome images; settings can govern whether and how those images are downsampled and compressed. See Adobe’s PDF Optimizer settings and PDF settings overview.
Choose lossless or lossy compression by image content and fidelity needs
Lossless: preserve image data
Lossless compression reduces the encoded size without removing image data. Adobe describes Flate (also called ZIP) this way: “The Flate method is lossless, which means it does not remove data to reduce file size and so does not affect an image’s quality.” Adobe PDF creation SDK documentation explains Flate and image downsampling.
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ZIP/Flate is a straightforward choice when retaining the image data is more important than achieving the smallest possible file. Adobe’s Optimizer guidance identifies ZIP as suitable for illustrations with solid or flat-color areas. Actual file-size savings depend on the source images and settings.
Lossy: trade some detail for a smaller encoding
Lossy methods discard image information. JPEG-style compression can be appropriate for photographs and other images with gradual color transitions, but stronger compression can visibly damage fine detail, edges, or gradients. Adobe recommends JPEG or JPEG 2000 for gradual color transitions and ZIP for flat-color illustrations; this is a practical content-based guide, not a universal quality ranking. Adobe’s PDF Optimizer settings describes these choices.
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JPEG 2000 options can also be configured for lossy or lossless compression; the method and settings matter, not just the format name. Adobe’s PDF creation SDK documentation describes the distinction.
Set image resolution for the destination
Resolution is a destination decision, not a universal preset. Adobe’s 2025 PDF settings overview gives these examples of image resolution for particular output devices. They are examples, not requirements for every printer or workflow:
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| Output device in Adobe’s example | Example image resolution |
|---|---|
| 300 dpi laser printer | 120 ppi |
| 600 dpi laser printer | 170 ppi |
| 1200 dpi imagesetter | 240 ppi |
| 2400 dpi imagesetter | 300 ppi |
Adobe notes that the resolution control in its documented settings ranges from 72 to 4000; that is the control’s range, not a recommended range for every PDF. Higher resolution can increase file size and processing time. For a print job, use the print provider’s requirements when available rather than treating these examples as a universal target. See Adobe’s PDF settings overview.
Scanned PDFs and PDF/A constraints
For scanned PDFs, Adobe Acrobat’s scanned-PDF tools include resolution (DPI) and image optimization controls. Adobe recommends JPEG 2000 for efficient color image compression and says ZIP is an option when lossless compression is preferred. Its guidance warns that JPEG 2000 is not recommended for PDF/A files. If the document must meet an archival or regulated conformance requirement, preserve that requirement and validate the resulting file in the intended workflow instead of optimizing only for size. Compatibility details can vary by PDF/A conformance and implementation. See Adobe’s guidance for improving scanned PDFs.
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A practical way to choose settings
- Identify the destination. Decide whether the PDF is mainly for screen reading, ordinary printing, or a specified production device or provider. Follow any stated delivery requirements.
- Inspect the source images. Determine whether they are photographs with gradual tones, flat-color illustrations, scans, or a mix. Check whether their current resolution is substantially higher than the destination needs.
- Set downsampling deliberately. If source images have more resolution than the output calls for, consider reducing it. Avoid lowering resolution without checking whether small details will remain legible at the intended scale.
- Choose the encoding. Prefer ZIP/Flate when preserving image data is the priority. For photographs or other gradual-tone images, consider JPEG or JPEG 2000 if lossy compression is acceptable. For flat-color illustrations, Adobe’s guidance favors ZIP.
- Account for compatibility. Check any required PDF version, PDF/A conformance, or reader/workflow limitations before applying a method. Do not let a smaller file override a required standard.
- Optimize a copy and inspect it. Save a separate copy, apply the settings, and compare the resulting file size with the original. Examine small text, edges, gradients, and photographs at the intended viewing or printing scale. If quality or compatibility is inadequate, return to the original and use less aggressive settings.
In Acrobat’s PDF Optimizer, Adobe documents an option to optimize images only when doing so reduces file size. This can help avoid applying an image change that makes the PDF larger; it does not replace checking visual quality or conformance.
Quick Recap
Best Value
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