How to compress a PDF to an exact size for an upload limit

Read the limit carefully first

An upload form that says "max 2 MB" is checking a number of bytes, and there are two ways to turn megabytes into bytes. Windows, most phones and this site count a megabyte as 1,048,576 bytes. Many websites count it as 1,000,000. A file of 2,050,000 bytes is "1.96 MB" in a file manager and over the limit on a form that means 2,000,000.

That gap is why people see "file too large" for a file their computer says is small enough. The safe habit is to aim for the stricter figure. iBuildPDF's size targets already do: "2 MB" is treated as 2,000,000 bytes, and "200 KB" as 200,000, so the result passes whichever way the form counts.

Also check what the form actually wants. Some portals limit each file, some the total of all your uploads, and some publish a limit that is not the one they enforce. Why an upload form rejects a PDF goes through the other reasons a form says no.

How to do it

On Compress PDF, choose Specific size and type the limit — 300 KB, 1.5 MB, whatever the form says. The common sizes have their own pages too: under 500 KB, under 1 MB and under 2 MB, where the size can also be changed.

What happens is a sequence of attempts, gentlest first:

1. The images are recompressed at progressively lower resolution and quality. Text, links and form fields are untouched, so the document is still a document. Many PDFs with photographs reach their target here.
2. Only if that is not enough, the pages are turned into images at progressively lower quality. This reaches almost any size, and it costs the selectable text — the result says plainly when it happened.

It stops at the first attempt that fits, which is the point: you get the best-looking file that meets the limit rather than the smallest file the tool could make. The whole process runs in your browser; the document is not uploaded.

Will it fit? Count kilobytes per page

Whether a target is realistic depends on what the pages are, and a few seconds of arithmetic answers it before you try.

Divide the limit by the number of pages that are photographs or scans. Pages of ordinary typed text cost almost nothing and can be ignored.

• 200 KB, 2 scanned pages: about 100 KB a page. Fine — a clear, slightly soft scan.
• 200 KB, 10 scanned pages: about 20 KB a page. Readable at a push for a typed letter, poor for anything with small print or a photograph.
• 200 KB, 40 scanned pages: 5 KB a page. Not readable at any setting.
• 2 MB, 40 scanned pages: about 50 KB a page. Usually fine.

A document made from text — exported from Word, not scanned — behaves completely differently. Forty pages of text is often under 200 KB before you start, and if it is not, the size is in images, fonts or drawings rather than in the words. Why some PDFs will not get smaller covers the files that resist.

When the honest answer is to split

If the arithmetic gives you a handful of kilobytes per scanned page, stop compressing. The tool will still produce the smallest file it can, but nobody will be able to read it, and a form that accepted an unreadable file has not helped you.

• Split the document. Most portals that limit a file's size accept several files. Split PDF divides it into parts; then compress each part to the limit.
• Leave out what is not required. A 30-page bank statement where the form needs the last three months is a job for extracting pages, not for compression.
• Re-scan instead. Scanning in black and white at 200 dpi rather than colour at 600 produces files a tenth of the size that are often sharper for text. Why a scanned PDF is so large has the numbers.

Check the result before you upload it

A file that fits is not automatically a file that works. Before you submit it:

• Open it and zoom to 100 %. Read the smallest print on the page that matters most — a reference number, a date, a signature. If you cannot read it, neither can the person reviewing your application.
• Check whether the text still selects, if the form will be processed automatically. A file whose pages were turned into images can look perfect and still fail a system that reads the text. The result tells you which kind of compression was used.
• Keep the original. Compression to a small target removes detail that cannot be put back. Save the compressed copy under a new name.