Reduce PDF DPI: what resolution your images need

DPI belongs to the pictures, not the PDF

A PDF has no single resolution. Text is stored as characters and a font, and lines and shapes as drawing instructions; both are redrawn sharp at any zoom, so they have no dpi at all. Only pictures have one: scans, photos, screenshots, artwork.

A picture’s dpi depends on how big it is drawn on the page. The same photo, 1200 pixels wide, is 400 dpi when placed 3 inches wide and 200 dpi when placed 6 inches wide. That is why “reduce the DPI of a PDF” really means: for every picture, keep only as many pixels as the size it is shown at needs.

Our compressor works exactly this way. It measures the dpi each picture is placed at, and shrinks only the pictures above the preset’s resolution (with a little slack, so a 120 dpi picture keeps its pixels under a 110 dpi preset rather than being resampled for nothing). Pictures already at or below the target are re-encoded only when that clearly saves space.

The squared rule

Resolution counts across and down at the same time. Halve the dpi and you halve the pixels in each direction, which leaves a quarter of them. Double it and you have 4 times as many.

We measured it on page 1 of our 10-page test scan (an A4 page at 300 dpi in colour, 1.7 MB as the scanner app saved it), re-encoded each time at the same JPEG quality (75):

Resolution	Page width in pixels	1 page
300 dpi	2480	1.0 MB
200 dpi	1654	533 KB
150 dpi	1240	330 KB
100 dpi	827	167 KB
75 dpi	620	100 KB

From 300 to 150 dpi the page got 3 times smaller, and from 150 to 75 dpi a little more than 3 times. That is less than the 4 times the pixel count predicts, because a smaller picture packs more detail into each pixel and JPEG spends more bytes on each, but no other setting comes close. JPEG quality is a finer adjustment on top: the Email preset’s 110 dpi at quality 65 gives this page 177 KB, about what 100 dpi at quality 75 gives.

What 72, 150 and 300 dpi look like

The useful way to think about dpi is pixels across the page, compared with the pixels of the screen it is read on. An A4 page is 8.27 inches wide:

• 72 dpi: 595 pixels across. Fine for a photo seen small, rough for scanned text: normal-sized letters still read at page width, small print breaks up, and zooming shows blocks.
• 110 dpi: 910 pixels across. Our Email preset. Scanned letters, forms and receipts read cleanly at page width on a laptop or a phone. Zoom in hard and the edges of letters soften.
• 150 dpi: 1240 pixels across. Our Web preset. More than most screens show at page width, so it looks sharp at normal zoom and holds up when the reader zooms in a little.
• 300 dpi: 2480 pixels across. Our Print preset. Stays crisp at 200 percent zoom and on paper. On a screen at page width it looks the same as 150; the extra pixels only appear when you zoom in or print.

The pattern: above the screen’s own pixel count, more dpi is invisible until someone zooms or prints. Below it, text in scans is the first thing to suffer.

Pick the dpi by destination

Where the file goes	Pick	In the tool
Email, chat, portal upload read on screen	96 to 110 dpi	Email preset
Shared for reading, may be zoomed	150 dpi	Web preset
Paper, archive, anything you may need later	300 dpi	Print preset
A portal that names a size in KB or MB	whatever fits	Type the size target

With a size target, the tool searches a fixed ladder of 8 settings: 300 dpi at quality 85, 200 at 80, 150 at 75, 110 at 65, 96 at 55, 72 at 45, 60 at 35 and 50 at 25. It keeps the gentlest one that fits, whatever the preset says, and tells you which it used. On our 10-page test scan, a 2 MB target kept 110 dpi (1.8 MB), a 1 MB target ended at 72 dpi (637 KB) and a 500 KB target at 60 dpi (366 KB); 5 phone photos under 2 MB kept 150 dpi (1.1 MB). You cannot type an exact dpi; the 3 presets and the ladder are the settings on offer.

Printing has its own rules (printer resolution, scaling, bleed). The guide to printing a PDF right covers them.

Downsampling is one-way

Reducing dpi throws pixels away. No tool can put them back: “upscaling” a 72 dpi picture to 300 only enlarges what is there, it does not recover the text a 300 dpi scan held. So:

1. Keep the original file. Our compressor writes a new file and never overwrites yours.
2. Compress the copy you send, for the destination it is going to.
3. If you need a higher-resolution version later, go back to the original or the source, not to the small copy.

When a portal’s rules cannot both be met

Some upload forms ask for a minimum resolution (“at least 200 dpi”) and a maximum size (“under 200 KB”) at once. For a multi-page colour scan, those 2 rules can contradict each other: on our test scan, 1 page at 200 dpi was already 533 KB at quality 75.

What helps, in order:

1. Send fewer pages. Extract only what the form asks for.
2. Go grayscale with Grayscale PDF, then compress. On scans of black-on-white paper the saving is modest (our test page went from 177 KB to 172 KB on the Email preset), but it can be the last few KB you need.
3. Scan again at the minimum the portal names, in grayscale, rather than at 300 dpi in colour.

Be careful with the settings here. Only the Print preset keeps pictures at 300 dpi; Web goes to 150 and Email to 110, both under a 200 dpi minimum. A size target keeps 200 dpi when that fits, but goes down to 50 dpi if it must, and the result says which it used. When a portal names both a dpi minimum and a size cap, cut pages and go grayscale before you cut resolution, and if that is not enough, scan again at the minimum rather than compressing below it.

What this tool does not do

It does not let you set an arbitrary dpi such as 240 or 125; you pick a preset or a size target. It does not raise resolution. It leaves 1-bit black-and-white scans as they are, since they are already compact. And text and vector drawings have no dpi to reduce, so a PDF without pictures barely shrinks; Why is my PDF so big shows how to check what your file is made of.

On a Mac, Preview has a built-in alternative: File, Export, then the Quartz Filter menu, Reduce File Size (Apple’s Preview user guide, checked on 11 October 2026). It is quick and offline, but you do not choose the resolution, and Apple’s guide notes the result may be of lower quality than the original. When you need a predictable dpi or an exact size, use the presets above.