An upload form says "maximum 10 MB" and your PDF is 34 MB. Or Gmail refuses the attachment, or turns it into a Google Drive link the other side's IT department won't let them open. You need the same document, in pieces, each under a limit.
Split by size does exactly that. You give it a maximum, it fills each file with as many whole pages as fit, and it shows you the real size of every part before you download anything. The work happens in your browser. Nothing is uploaded.
Why you can't just divide by the page count
The obvious approach is arithmetic. 34 MB across 170 pages is 0.2 MB a page, so 50 pages per file should come out at 10 MB. In practice it almost never does.
Pages in a PDF aren't the same size. A page of typed text might take 20 KB. A page with a full-colour photo might take 2 MB. A scanned contract can be 300 KB a page for the typed clauses and three times that for the page with a stamped, signed annex. Divide evenly and one part comes out at 4 MB while the next is 15.
There's a second catch. Fonts, logos and other resources are stored once in the original and shared by every page that uses them. When you split, each new file needs its own copy of whatever its pages use. So five parts of a 34 MB file usually add up to more than 34 MB, and a part's size isn't just the sum of "its" pages.
That's why the tool doesn't estimate. For each part it builds a real candidate file in memory, checks its size, and adjusts. It starts from the length of the previous part, doubles the page count until the file goes over the limit, then keeps halving the difference until it finds the largest run of pages that fits. The size you see next to each part is the size of the file you'll get.
Splitting a PDF by size step by step
- Open Split by size and drop your PDF on the upload box. A password-protected file asks for its password first.
- Next to "Max size per file:", click a preset (1, 5, 10 or 25 MB) or type your own number. The KB and MB buttons switch the unit, and the presets become 100, 250 and 500 KB.
- Wait for "Calculating split points". A big file can take a while, longer on a phone.
- Read the list. Each row shows something like "Part 2: pages 48-91" and the actual file size.
- Click the button in the header, which says how many files you'll get, for example "Split into 4 file(s)".
The parts are named yourfile-part1-pages-1-47.pdf and so on. More than one part downloads as a ZIP ending in -split.zip.
If the PDF has bookmarks, a "Keep bookmarks" checkbox appears. Ticked, each part keeps the bookmarks that point to its own pages. Unticking it can let a few more pages fit into each part, because the bookmarks take a little space too.
The smallest limit you can set is 10 KB, and the default when you open the tool is 1.5 MB.
What "MB" means here, and why to leave headroom
The tool counts 1 MB as 1,024 KB, and 1 KB as 1,024 bytes. So a 10 MB part can be up to 10,485,760 bytes. That's the traditional way and it's what Windows File Explorer shows. But some websites count 1 MB as exactly 1,000,000 bytes, and to them that same file is 10.49 MB, which is over a 10 MB limit.
You won't know which way a portal counts. Set the limit a little under the real one, 9.5 instead of 10, and the question goes away.
Email needs even more room. Gmail allows up to 25 MB of attachments per message and Outlook.com 20 MB, but attachments are encoded for sending, which makes them roughly a third bigger in transit, and the recipient's mail server may have a lower limit than yours. For email I'd set the limit well under the official number and send the parts in separate messages. The 25 MB preset is there because it matches the stated limit, not because it's the safe choice.
When one page is bigger than the limit
Splitting never cuts a page, and it can't make a page smaller. If a single page is already over the limit (a high-resolution photo, a scanned map, an embedded poster) it's saved as a file of its own and marked in orange, with the message "Some pages are larger than the size limit on their own. Each of them is saved as a separate file."
That file will still be too big for your upload form. Run it through Compress PDF, which re-encodes the images, or get the whole document down first.
Should you compress instead
Usually, try compression first. Most oversized PDFs are oversized because of photos or scans saved at far more resolution than a screen needs, and Compress PDF can often shrink a file like that a lot with no difference you'd notice on screen. One file is easier for everyone than four. The guide on making a PDF small enough to email explains the settings.
Split by size makes sense when:
- the document is already compressed and still too big, such as a long scanned file;
- the portal accepts several files but caps each one;
- you mustn't change the content at all, because splitting leaves every page exactly as it was, at the same quality.
You can also do both. Compress first, then split what's left.
If you need the parts to follow chapters rather than sizes, this isn't the right tool. Split by size only looks at bytes, so a part can end in the middle of a section. Split PDF lets you pick the split points yourself.
Before you send anything, open the last part and check its final page is the last page of the document. If it is, nothing fell off the end.
Also useful: Compress PDF · Split PDF
