PDF file sizes in KB and MB, explained with real files

The units, with a PDF for each

Unit	Bytes (decimal)	Bytes (Windows)	A PDF that size
1 byte	1	1	1 character of text
1 KB	1,000	1,024	a short page of typed text, before fonts
100 KB	100,000	102,400	our 500-page text file (117 KB), or a clean greyscale scan of 3 pages
1 MB	1,000,000	1,048,576	6 scanned pages after the Email preset
10 MB	10,000,000	10,485,760	5 or 6 pages scanned at 300 dpi in colour
1 GB	1,000,000,000	1,073,741,824	an archive of hundreds of high-resolution scans

The examples come from our test files, measured on 11 October 2026: a 300 dpi colour scan is about 1.8 MB a page, the Email preset brings it to about 170 KB a page, and a clean 200 dpi greyscale scan is 34 KB a page. Typed text is so small that page count barely matters; the size is in pictures, which is the subject of why is my PDF so big.

Why 1,000 and 1,024 both exist

Computers count in powers of 2, and 2 to the 10th power is 1,024, close enough to 1,000 that early software borrowed “kilo” for it. Storage makers and the metric system kept kilo as exactly 1,000. Both are still in use:

• Windows File Explorer counts 1 KB as 1,024 bytes.
• Mac and iPhone count 1 KB as 1,000 bytes. Apple’s support page on how storage is measured says iOS 10 and earlier, and OS X Leopard and earlier, used the binary count, so anything newer uses the decimal one (checked on 11 October 2026).
• Upload portals do whatever their programmer chose. Some count in 1,000s, some in 1,024s, and few say which.

At small sizes the gap is small, 2.4 percent per KB. At MB it is about 4.9 percent, so a 10 MB limit can mean 10,000,000 or 10,485,760 bytes.

Check the exact size on any device

Windows

1. Right-click the PDF in File Explorer and choose Properties.
2. Read the Size line. The number in brackets is the exact count, for example “98.1 KB (100,500 bytes)”.

Mac

1. Select the PDF in Finder and press Command and I (or right-click, Get Info).
2. The Size line shows the byte count, counted in 1,000s.

iPhone and iPad

1. In the Files app, touch and hold the PDF.
2. Choose Get Info. The size is rounded, in decimal units.

Android

1. In your file manager (Files by Google or the one your phone came with), open the PDF’s menu.
2. Choose File info or Details, depending on the app. The size is rounded; which unit it uses varies by app, so leave a margin.

A phone shows a rounded size, which is fine for an email limit and risky for a strict 100 KB check. When the margin is thin, check on a computer, or use a target with room to spare.

Reading a limit correctly

“Max 100 KB” can mean 100,000 or 102,400 bytes, and the form rarely says which. You do not need to know:

1. Take the stated number and assume the stricter reading: 100 KB is 100,000 bytes, 2 MB is 2,000,000 bytes.
2. Stay under that byte count. If all you can see is a rounded size, as on a phone, leave a few percent more.
3. Check the result’s exact size before you upload.

A file that passes this way passes both counts. If the form also caps the number of files or pages, that is a different limit; the portal limits guide lists the common ones.

Page count says little

A size limit is about bytes, and pages are a poor guide to bytes. Our 500-page text file is 117 KB; 1 page of our colour scan is 1.8 MB. 100 typed pages can weigh less than 1 scanned page. If a portal gives a per-page limit (some do, like 200 KB a page), that is really a limit on how heavy each scan can be. The guide what size should a PDF be has the per-page numbers by content.

How our numbers read

The compressor’s size target counts 1 KB as 1,000 bytes, the stricter reading. When you type 100 KB, the result is at most 100,000 bytes, so it passes a portal whichever way the portal counts; when a file cannot get that small, the result says so and how close it got. A file already under the target comes back untouched. Compress to 100 KB and the other size pages work the same way.

One honest caveat about the readout: the file sizes on the result card are rounded and count 1 KB as 1,024 bytes, the way Windows does. When you set a target, the report lines count in 1,000s, like the target, so the same file can read 208 KB on the card and “Under 500 KB (213 KB)” in the report. Neither changes whether the file passes, because the target itself is checked in exact bytes. When a portal is strict, check the downloaded file’s exact byte count with the steps above, the same number the portal’s check sees.

The compressor also does not decide what a portal will accept beyond the size: a page limit, a file-type rule or a rule about scans in colour is outside what it can check. Everything runs on your device, and the file is never uploaded, so checking and re-running costs nothing.