How small is 100 KB for a PDF?
A page of text in common fonts takes a few kilobytes, so many text documents are under 100 KB before any tool touches them. Pictures are what make a PDF big. A colour photo of a full A4 page cost about 336 KB at Balanced in our test, and a scan is often more than a megabyte. To fit 100 KB, the pictures have to shrink to about the size of a small screen: at 100 dpi an A4 page is 827 by 1,169 pixels, and at 56 dpi, the lowest setting the tool uses, it is 463 by 655.
That is why the outcome depends on how many different pictures there are, not on how many pages. In our tests, the PDF holding one colour A4 scan on four pages fitted 100 KB with its picture at about 125 dpi, which is a little soft but readable for body text. The six page photo PDF did not fit: at 56 dpi it still came to 121 KB. A mostly text PDF that was saved without compression is the easy case: a 40 page one went from 335 KB to 72 KB, with the text untouched.
How the tool finds the biggest size that fits
The tool compresses the PDF once at your level. That shows how much of the file is pictures and how much is text, fonts and structure. Only the pictures can change, so it predicts the resolution at which they would fill the space left under 100 KB, and tries it. Every try is a full compression, and the next one is predicted from what the last one showed, so it takes at most five tries and lands close under the limit instead of far below it.
Resolution and picture quality are lowered together, because a sharp picture with too few pixels and a blurry picture with many pixels both look worse than a balanced mix. The note on the result tells you the resolution the pictures ended up at, measured on the file you get.
When 100 KB is out of reach
- Several photos or full page scans. Every page needs its own picture, and 100 KB shared between them leaves very little for each. Remove the pages you do not need with Split PDF or Delete PDF Pages, or ask whether the form takes a larger file.
- Many different pictures. Our four page PDF with a dashboard screenshot and a photo on each page fitted at about 137 dpi, because the same two pictures were used on every page. Six different full page photos did not fit, even at the lowest setting.
- Scans of text. A scan is a picture of the page, so it gets softer as the limit goes down. If the form allows it, scan again in black and white at 150 dpi. Such scans are very small to begin with, and this tool leaves them as they are.
If the result is just over 100 KB, take out one heavy page, or start again from the original file rather than from one that was already compressed.