PDF compression guides
DPI vs PPI in PDFs: What Resolution Means Before You Compress
Measure raster images at their placed size, distinguish screen pixels from printer dots, and choose compression around the document's real use.
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PPI describes how many raster-image pixels occupy an inch at a chosen display or placed size; DPI describes the dots a printer can place on paper. A PDF normally does not have one universal DPI because a page can mix live text, vector graphics, and several images at different effective resolutions. Before compression, evaluate each important raster image and the document's intended use.
What do DPI, PPI, pixels, and effective PPI mean?
| Term | What it measures | How it matters in a PDF |
|---|---|---|
| PPI | Raster pixels per inch at a defined physical or placed size. | Useful for judging whether an image has more pixel detail than its placed size requires.1 |
| DPI | Dots per inch produced by a physical output device such as a printer. | Relevant to print output, but not a universal measurement of every object on a PDF page.1 |
| Pixel dimensions | The width and height of a raster image in pixels. | The fixed pixel count used with placed size to calculate effective PPI. |
| Placed size | The physical width and height at which an image appears on the PDF page. | Changing placed size changes effective PPI without changing the image's pixel dimensions. |
| Live text and vector graphics | Shapes and glyphs described mathematically rather than as a fixed pixel grid. | They do not acquire one image PPI merely because they share a page with raster images. |
How do you calculate effective PPI for an image in a PDF?
effective PPI = pixel dimension ÷ placed dimension in inchesA 2400-pixel-wide image placed 8 inches wide has an effective resolution of 300 PPI: 2400 ÷ 8 = 300.
Calculate width and height separately when proportions or scaling differ. The lower result is often the practical limit for detail in that direction. Resizing the image larger on the page lowers effective PPI; placing the same pixels in a smaller area raises it.
Why does a PDF usually not have one DPI value?
A PDF is a container for page objects, not one flat bitmap by definition. One page can hold searchable text, a vector logo, a 300 PPI scan, and a small image placed at a different effective PPI. Reporting one whole-file DPI hides those differences and cannot by itself prove reading, printing, or preservation quality.
What resolution is appropriate before compression?
| Intended use | Starting point | Required verification |
|---|---|---|
| Routine screen reading | Inspect at the real reading size instead of assuming that 72 or 96 PPI describes the whole PDF. | Small text, diagrams, screenshots, zoom behavior, and searchability. |
| Ordinary office document scan | Pennsylvania State Archives lists 300 PPI as common for ordinary office documents. | Text edges, handwriting, stamps, page completeness, and the receiving office's rules.1 |
| Archival or detail-sensitive material | Use the applicable FADGI material category and project specification rather than a generic web target. | Capture quality, color, fine detail, metadata, and preservation-master requirements.3 |
| Scanned textual record | Follow the record owner's scanning and access-copy requirements. | Legibility, completeness, orientation, and retention of meaningful marks. |
How can PDF compression change image resolution?
PDF optimization can downsample images and apply lossy or lossless image compression. Downsampling removes pixels, so effective PPI falls when the placed size stays the same. Lossy compression can also introduce visible artifacts without changing the nominal pixel dimensions. Adobe therefore separates image settings by color, grayscale, and monochrome content rather than treating the PDF as one uniform image.2
- Identify the raster images that carry meaningful detail.
- Calculate or inspect their effective PPI at the placed size.
- Choose a compression route appropriate to screen, office, print, or preservation use.
- Open the downloaded PDF and inspect the actual detail, not only the reported file size or resolution metadata.
FAQ
Is a 300 DPI PDF always high quality?
No. The label may refer to a scan setting, an export setting, or one image. A PDF can mix objects at different effective resolutions, and quality also depends on focus, compression, color, and intended use.
Does compressing a PDF always reduce PPI?
No. Removing redundant structure or using lossless methods may reduce bytes without changing pixel dimensions. Downsampling does reduce pixel count, while lossy compression may reduce quality even when dimensions remain unchanged.
Should I use DPI or PPI when evaluating images inside a PDF?
Use effective PPI for raster images at their placed size. Use DPI when discussing the physical printer or output process, then verify the final print or screen result for its intended purpose.
Sources
- Pennsylvania State Archives — What standards should I follow?government technical guidanceSource reviewed:
- Adobe Acrobat — PDF Optimizer settingsofficial technical documentationSource reviewed:
- FADGI — Technical Guidelines for Digitizing Cultural Heritage Materialsgovernment technical guidanceSource reviewed: