How to compress a PDF to a specific size (and why some won’t shrink)

You’ve filled in the application, scanned the documents, and the portal says “Maximum file size: 1 MB”. Your PDF is 8 MB. This guide explains how to get under a specific limit, such as 2 MB, 1 MB, 500 KB or 200 KB, without making the document unreadable, and why a few PDFs stubbornly refuse to shrink however hard you try.

Why upload limits exist

Job sites, visa services, university admissions systems, insurers and council portals all store what you send, often for years, and process thousands of applications. Small limits keep their storage and email systems manageable. They’re usually set with a typical document in mind (a few pages of typed text) rather than a stack of high-resolution phone scans, which is why scanned documents run into trouble far more often than files exported from a word processor.

The limit is almost always on the file, not the page count. So the goal is to spend your size budget wisely: keep text sharp, and make photos and scans only as detailed as they need to be on screen.

What makes a PDF big

A PDF is a container. Inside it there can be text with its fonts, vector drawings (lines, shapes and charts), and images. In nearly every oversized PDF, the images are to blame.

• Scanned pages are images. A scanner or scanning app photographs each page. An A4 page scanned in colour at 300 dpi is about 2480 × 3508 pixels, nearly 8.7 million pixels per page. Even compressed, that is easily several hundred kilobytes per page, and more at 600 dpi.
• Photos are bigger than they look. A modern phone photo may be 4000 pixels or more across. Placed on an A4 page, that’s far more detail than any screen will show.
• Text is tiny. A page of typed text, stored as real text with an embedded font, is often just a few kilobytes. Fonts add a little; a document that embeds many complete fonts can carry a few hundred kilobytes of font data, but that’s rarely the main problem.
• Duplicates and leftovers. Logos repeated on every page can be stored many times over, and some files carry embedded page thumbnails or editing history.

That’s why compression tools concentrate on images. Resizing a scan from 300 to 150 pixels per inch cuts its pixel count to a quarter, and re-saving it as a JPEG at a sensible quality shrinks it further. On a screen, 150 ppi still looks sharp for normal reading.

Step by step: hit a target size

Here’s how to do it with Compress PDF, which runs in your browser so the document never leaves your device. That matters when it’s a passport scan or a bank statement.

1. Open the PDF. Drop it on the tool or choose it from your files. You can add several files to compress as a batch.
2. Choose Target size. Enter the portal’s limit, or tap one of the shortcuts: 200 KB, 500 KB, 1 MB, 2 MB or 5 MB. Aim slightly below the limit if you can, because some portals count a megabyte as 1,000,000 bytes and others as 1,048,576.
3. Compress. The tool tries the Light level first, then Balanced, Strong and finally Maximum, and stops at the first one that fits, so you get the best quality that meets the limit.
4. Check the result. You’ll see the size before and after. If no level fits, you get the smallest version we could make, marked “best achieved, above target”, and the sections below explain what to try next.

Prefer to choose the quality yourself? Pick a level directly: Light keeps images up to about 200 ppi (good for printing), Balanced about 150 ppi (sharp on screen), and Strong about 100 ppi (smallest, softer images). Two extra options help scans: Convert images to grayscale, which is ideal for black-and-white letters and forms, and Remove metadata, which strips the title, author and editing history.

What’s realistic for each limit

A useful way to think about it is a per-page budget: divide the limit by the number of pages. A 1 MB limit for a 10-page document gives each page about 100 KB. Typed pages need only a few KB each; scanned pages need much more. The table below is a rough guide to typical documents, not a promise: results vary a lot with scan quality, colour and content.

The 200 KB row is where expectations most often clash with reality. Two scanned pages at 100 KB each will be readable in grayscale but soft; ten scanned pages at 20 KB each will not be. If a portal really wants 200 KB for a long scanned document, the honest answer is usually to send fewer pages or ask whether it accepts several files.

Getting under 100 KB

Some forms, particularly government and exam portals, ask for 100 KB or less per document. That’s tight but often achievable for a single page, and the per-page budget tells you quickly whether it’s realistic: 100 KB for one page is workable; 100 KB for ten scanned pages leaves about 10 KB a page, which isn’t enough for a legible scan.

1. Send only the pages asked for. If the portal wants your certificate, upload the certificate page, not the whole pack. Extract pages does this in a few clicks.
2. Tick Convert images to grayscale. For a black-and-white document it makes little visible difference and saves a lot.
3. Use Target size and enter the limit. Our tool counts 1 KB as 1,024 bytes. If the portal counts in thousands (100 KB = 100,000 bytes), enter 95 KB to leave a margin.
4. Check legibility. At this size the tool may need its Maximum level, which keeps images at up to about 72 ppi. Zoom in on names, numbers and signatures before you upload.

If the smallest version is still over the limit, the remaining options are the ones in the next sections: fewer pages, a cleaner rescan, or splitting the document if the portal accepts several files.

Why some PDFs won’t shrink

Sometimes the result is only a few per cent smaller, or the tool returns the original with the message that the PDF is already well optimised. That isn’t a fault. It happens for predictable reasons:

• It’s mostly text. Text and fonts are already compact, and lossy image compression has nothing to work on. A 300 KB typed document might only lose a few kilobytes of structure.
• The images are already small. If the scan was saved at low resolution with heavy JPEG compression, there’s little left to remove without making it unreadable, and re-compressing a JPEG at the same size can even make it bigger. We keep the original image in that case.
• The images use formats we leave alone. Some PDFs store images as CMYK (print-ready colour), 16-bit, JBIG2 (a black-and-white scan format) or JPEG 2000. Browsers can’t reliably re-encode these without risking colour shifts or damage, so we skip them and the result tells you how many were left unchanged. JBIG2 scans are usually very compact already.
• It’s full of vector artwork. Architectural drawings, maps and complex charts are made of thousands of lines and shapes rather than pixels. They stay sharp at any zoom, but there’s no image to downsample.

Whatever the reason, the tool never hands you a file larger than the one you started with. If it can’t save anything, you get your original back.

When it still won’t fit

If Maximum still doesn’t reach the limit, these options, roughly in order of preference, usually get you there:

1. Send only the pages that are needed. Portals often ask for “proof of address”, not the whole twelve-page statement. Use Extract pages to keep just the relevant pages, then compress again.
2. Split the document. If the portal accepts several uploads, use Split PDF to make two or three files that each fit.
3. Try grayscale. Colour images carry three channels of information; grayscale carries one. For letters, forms and certificates, this alone can halve the size.
4. Rescan at a sensible setting. If you control the scanner, 200–300 dpi in grayscale is plenty for documents. Scanning at 600 dpi colour and then compressing throws away most of what was captured.
5. Last resort: rebuild from images. You can turn pages into images with PDF to JPG at a low resolution and rebuild a PDF with JPG to PDF. This shrinks almost anything, but text becomes part of the picture: it can no longer be selected or searched, and small print gets blurry. Use it only for scans, where the text was already a picture.

Before you upload

A quick check saves an awkward rejection email later:

• Open the compressed file and zoom to 100%. Can you read the smallest print, such as reference numbers, dates of birth and signatures?
• Check that the page count is what you expect and every page is the right way up. Rotate PDF fixes sideways pages without adding any size.
• Give the file a clear name, such as Surname-Firstname-passport.pdf; many reviewers download dozens of files.
• If the portal rejects a file that looks small enough, check whether it measures sizes in thousands or in 1,024s, and aim a little lower.

Most of the time, though, it’s simple: open the file in Compress PDF, choose Target size, and let the tool find the best quality that fits.