Lossless vs lossy PDF compression: how to shrink a PDF without losing quality
A heavy PDF is slow to send, slow to open online, and costly to store. To shrink it wisely, you need to understand where its weight comes from and how the two main compression families work: lossless and lossy. This guide explains the mechanisms, then the concrete strategies for a light yet readable file.
Why do PDFs get large?
A PDF's weight almost never comes from its text. A full page of text is only a few kilobytes. Images are what inflate a document: photos, screenshots, high-resolution logos, and above all scanned pages, which are in fact large pixel-by-pixel images.
Scans are the most common culprit: a page scanned at 600 DPI in color can weigh several megabytes on its own, because the whole page is stored as a complete photograph rather than as selectable text.
Other elements add up too: embedded fonts (especially when they are not subsetted), metadata, thumbnails, hidden layers, duplicated objects, and form data. Each is small on its own, but together they bloat the file unnecessarily.
Lossless vs lossy compression: the real difference
Lossless compression reorganizes and encodes the data more efficiently without discarding any of it. The restored file is byte-for-byte identical to the original. It acts on the PDF structure, text, fonts, and metadata, producing modest but completely invisible savings.
Lossy compression, by contrast, permanently removes information judged hard to perceive, mainly inside images (the same idea as the JPEG format). It delivers far larger reductions, but every pass destroys detail in a way that cannot be undone.
At a low ratio the loss is invisible: an illustrative photo still looks sharp on screen. At a high ratio, artifacts appear: halos around scanned text, color banding, blurry edges. The rule is simple: keep vector and text content lossless, and use lossy compression for photographic images with a reasonable setting.
DPI and downsampling: screen vs print
DPI (dots per inch) measures the pixel density of an image inside the PDF. The higher the DPI, the more detailed and the heavier the image. Shrinking a scan often comes down to lowering its DPI, a process called downsampling.
The trick is to match the DPI to the actual use. For on-screen reading or online sharing, about 150 DPI is plenty, because a screen cannot show more detail than that. For quality printing, aim for around 300 DPI, the standard resolution for a crisp result on paper.
Keeping 600 DPI for a document meant only for screens changes nothing the reader can see, yet it multiplies the file size. Matching DPI to the target is therefore the single most effective lever to shrink a PDF with no perceptible loss.
Practical strategies for good compression
First rule: always compress from the original file, never from an already-compressed PDF. Re-compressing a lossy file stacks up artifacts and degrades quality with every pass, often without saving much extra space.
For a text document, a contract, or lecture notes with no color photos, converting to grayscale with the grayscale tool removes unneeded color information and cuts weight noticeably while keeping perfect readability.
For a file meant to be viewed online, the web-optimize tool applies linearization (fast web view): the PDF starts displaying before it is fully downloaded, improving the web experience. You can combine it with compression for a light, smooth result.
Finally, always keep a high-quality copy of the original and compress a working version. That way you can regenerate a clean file if a finer rendering is ever needed.
How to compress with the compress tool
The compress tool shrinks your PDF by optimizing its structure and downsampling images to a suitable DPI, while keeping text sharp and selectable.
Upload your original PDF, choose a compression level to match your need (online sharing or printing), then run the process and download the lighter file. Processing happens within the tool's flow, without durable storage of your documents.
Then compare weight and visual quality. If the result suits you, you are done; otherwise start again from the original with a less aggressive setting rather than re-compressing the already-reduced file.
Frequently asked questions
How do I reduce a PDF without losing quality?
Start from the original file and downsample images to the DPI that matches the use (about 150 DPI for screen, 300 DPI for print). Text is vector-based and stays sharp. The compress tool applies these settings automatically.
What is the difference between lossy and lossless compression?
Lossless compression reduces size without discarding any data, so the file stays identical. Lossy compression permanently removes barely perceptible image detail to save far more space, at the cost of artifacts if pushed too far.
Why is my PDF still large after compression?
Often because it contains high-resolution scans, or because it was already compressed once. Re-compressing yields little. Go back to the original and lower the images' DPI, or convert it to grayscale with the grayscale tool.
What is the best DPI for a PDF?
It depends on the destination: about 150 DPI is enough for on-screen reading or online sharing, while 300 DPI is recommended for crisp printing. Beyond that, size grows with no visible benefit on screen.