Why is my PDF so large?
A twenty-page contract exported from Word can weigh under 300 KB. A single page photographed with a phone can weigh 4 MB. Same format, same page count in the first case being fourteen times the content — and the small one is the long one. Once you know why, you can predict which of your files will shrink, which will not, and what to do about the ones that will not.
Compress PDF →Text weighs nothing. Pictures weigh everything.
A PDF is a container, and what goes inside it comes in two very different forms.
Text pages store characters, font references and coordinates — the instruction “draw the letter A here, in this font, at this size”. That is a handful of bytes per character. Twenty pages of it is a rounding error, which is why documents exported from Word, Excel, a browser or an accounting system are usually measured in hundreds of kilobytes.
Image pages store pixels. A scanned document is nothing but images: one full-resolution photograph per page, stacked as many times as there are pages. Nothing on that page is text as far as the file is concerned, even though you can read it perfectly well.
So the first question about any heavy PDF is not “how do I compress it” but “what is inside it”. The answer determines everything that follows.
The five-second diagnosis
Open the file in any viewer and try to select a line of text with your mouse.
The text highlights. Your PDF has a text layer. If it is still large, the weight is coming from something specific: embedded photographs, a full-page background image, a scanned logo repeated on every page, embedded fonts, or an attached file. Compression will help less than you hope, because most of the document was already efficient.
Nothing highlights. It is a scan or a set of photos. The weight is exactly what you would expect, and this is the case where compression does dramatic work.
A rough guide to what different sources produce per page:
| What produced the page | Typical weight per page | Will compression help? |
|---|---|---|
| Exported from Word or Excel | 20-50 KB | Barely — there is nothing to squeeze |
| Black-and-white scan, 300 DPI | 0.3-1 MB | Yes, several times smaller |
| Color scan, 300-600 DPI | 1.5-5 MB | Yes, the most dramatic case |
| Phone photo of a page | 2-5 MB | Yes, and cropping helps too |
| Presentation with photographs | 0.5-3 MB | Yes, but watch the image quality |
| Drawing or vector graphic | 0.1-2 MB | No — the vector would become a picture |
The size was decided before you opened the file
For scans, almost all the weight is set by settings chosen at scan time, and none of them can be recovered afterwards. Three matter:
Color mode. A color scan of a black-and-white page stores three channels of information about a document that has one. Switching a scanner to grayscale typically halves the size; true black-and-white mode can cut it by a factor of five. Nothing is lost that was ever on the paper.
Resolution. 600 DPI holds four times the data of 300 DPI. It is the right choice for photographs and archival work and pointless for a typed page — no reader will ever see the difference on screen or in print.
Camera versus scanner. A phone camera produces an 8-12 megapixel image of a sheet of paper, which is far more data than a 300 DPI flatbed scan, plus noise and uneven lighting that compress badly.
If you still have access to the original document or the scanner, redoing the scan at sensible settings beats every compression trick available. It is also the only route that keeps the pages sharp.
What compression can and cannot do
Compress PDF works by re-drawing every page as an image at a chosen resolution and assembling those images into a fresh document. Three levels are available: Light at 150 DPI for documents that will be printed, Medium at 120 DPI as the everyday default, and Strong at 96 DPI for tight upload limits.
What it does well: scans, photographs, brochures and photo-heavy presentations commonly drop by 60-90%. Page size, page order and the printed appearance are preserved exactly.
What it cannot do: make a text-only PDF meaningfully smaller. There were no images to re-encode, and the result card will say so honestly rather than handing you an identical file. It also cannot hit an exact target size on demand — the final number depends on what is printed on your pages.
What it costs: everything living underneath the visible layer. Selectable text, clickable links, bookmarks and fillable form fields become part of the picture. For a scan that changes nothing, because there was never a text layer. For a document exported from Word it is a real downgrade — and there, re-exporting from the original source file gives you a small file with the text intact and no compression at all.
Four fixes that are not compression
Before you trade quality for kilobytes, try the changes that cost nothing:
- Delete pages nobody asked for. Blank reverse sides from duplex scanning routinely double a document's length. Delete PDF pages removes them with zero loss.
- Crop the margins. A scanned A4 page can be a third empty white space, and white space is stored as pixels like everything else. Crop PDF trims every page with the same numbers in one pass.
- Send fewer pages. If only three pages are relevant, extract those three and send them. This is usually faster than any compression and keeps full quality.
- Re-export from the source. If the PDF came from a Word file you still have, export it again; if it started life as a spreadsheet, rebuild it with Excel to PDF. A file built from text rather than from a scan of text is smaller by an order of magnitude and stays searchable.
One last, unglamorous option: some documents are simply large, and the right answer is a link rather than an attachment. A 200-page scanned archive is not a mail attachment problem to be solved with a compressor — it is a file to put in cloud storage and share by URL.
Common questions
Why is my scanned PDF so much bigger than a text one?
Because a scan is a photograph of every page rather than the text of every page. A text PDF stores characters and positions, which cost a few bytes each; a scan stores millions of pixels per page regardless of how little is printed on it.
My PDF has text but is still huge. Why?
Look for embedded images: a photograph, a full-page background, a high-resolution logo repeated on every page, or a chart pasted as a picture. Fully embedded font families and attached files also add weight. Compression helps less here, and re-exporting from the source document usually helps more.
How much smaller can I expect the file to get?
Scans and image-heavy PDFs commonly shrink by 60-90%. Text-only PDFs may not shrink at all, because there are no images to re-encode. The before-and-after card shows the real numbers rather than a promise.
Does compressing damage the document?
It changes it. Pages are rebuilt as images at a lower resolution, so fine print and stamps soften at the stronger levels, and selectable text, links and form fields stop existing. Page size, order and appearance are preserved. Always keep the original.
Is it better to compress or to scan again?
Scan again, if you can. Grayscale at 300 DPI instead of color at 600 DPI produces a small, sharp file with a genuine text-free image and no quality trade-off after the fact. Compression is the fix for a file you cannot recreate.
Does the tool upload my file to check its size?
No. The PDF is opened, measured, re-rendered and rebuilt by your own browser on your own device. Nothing is transmitted at any stage.