How to reduce PDF file size

The three things that make a PDF big

A PDF is heavy for one of three reasons. Find yours before you set a number, because the reason decides how much weight comes off.

1. Scanned pages. Every page is a photograph of paper, so the file weighs what the photographs weigh, and the text on them is not text at all.
2. Placed images. A report with eight camera photos in it carries eight camera photos, at a resolution no printer and no screen will ever use.
3. Embedded fonts and leftover structure. A font program usually ships every glyph it was designed with, and a document usually draws a few hundred of them. Old revisions and orphaned objects sit in the file after edits and never get cleaned out. This one is real but it is rarely the whole story.

Tell them apart in five seconds. Open the file and try to select a line of text with the cursor. If the text highlights, the pages are not scans and the weight is in the pictures or the fonts. If nothing highlights, the page is an image and the scan is the file.

What the tool does about each one

The compress tool runs four methods against your file at the same time and keeps whichever result is smallest and still passes its check. The structural method rebuilds the document into a fresh file, dropping orphaned objects and old revisions. The image method re-encodes the placed images at 150 dpi with mozjpeg at quality 75. The font method subsets each embedded TrueType down to the glyphs the pages actually draw.

The fourth is the raster method, and it is the only one that touches the page itself. It renders each page at 150 dpi and redraws the page's own text invisibly on top, so the words stay selectable and searchable even though the picture underneath is now an image. It starts only when no other method reached your number, and it gives up early when its own projection cannot beat a candidate another method already holds.

On the public run of 843 cases, each one file at one target, the structural method won 173, the font method 79, the image method 230 and the raster method 361. Which one wins on your file depends on which of the three problems it has, and the result names the winner.

Type a size, not a quality level

Most tools give you a slider named low, medium and high, which tells you nothing about the file you get. This one takes a number in KB and works toward it. One KB is 1,024 bytes here, so a 200 KB target means the output lands under 204,800 bytes.

If an upload form counts a kilobyte as a flat thousand bytes, its cap is 4,800 bytes tighter than that. When a file that reads as the right size still gets refused, set the target about 3% lower and run it again.

How much usually comes off

Across the 843 public cases the median reduction was 72.5% of the source bytes and the top tenth reached 97.1%. No output came back larger than the file it started from.

Document kind decides the size per page. At the smallest setting a text page comes down to about 14 KB, a page of text with pictures to about 37 KB, a fax-era scan page to about 29 KB and a phone photo of a page to about 18 KB. Multiply by your page count and you know what number to ask for.

Check the result before you send it

Nothing ships unread. Every candidate is opened again and its text is compared against the source, and any candidate that lost more than 2% of the non-space characters is thrown away rather than offered to you. On the public run, every one of the 554 cases that carried text kept at least 99.99% of its characters.

The result names the size it reached and the method that won, so you know what you are sending before you send it.

From here, the quality guide covers which methods cost you nothing and which ones cost you detail. The compress hub lists the rest of the cluster, including phones, very large files and merged documents.

Common limits

Every cap a form is likely to name has its own page, and each page says what the number means in bytes and how many pages fit under it.

The two people ask for most are the mail attachment cap, where you compress a PDF to 1 MB, and the government upload field, where you compress a PDF to 150 KB. The rest of this cluster covers the cases the number alone does not: changing a PDF to a specific size when a form names a floor or a ceiling, compressing a large PDF when memory is the limit, compressing on a phone, and merging first and compressing once when the document arrives in parts.