Why Is My PDF File So Large?
Short answer
Divide the file size by the number of pages. Under 100KB per page means the PDF is already efficient. Over 1MB per page means images, and almost always scanned pages stored as full-resolution photographs. That single calculation tells you which fix will work and which will waste your time.
On this page
Before compressing a PDF, it is worth thirty seconds to find out why it is large. The reason matters, because the three causes need completely different fixes and applying the wrong one either does nothing or destroys the document.
There is one calculation that sorts this out.
Divide the size by the page count
Open the file properties and note the size, then divide by the number of pages.
| Size per page | What you have | What will fix it |
|---|---|---|
| Under 100KB | Text and vectors — already efficient | Nothing. This is as small as it gets. |
| 100–300KB | Text with some graphics, or many fonts | Lossless rebuild |
| 300KB–1MB | Compressed photos, or print-resolution graphics | Downsample images |
| 1–5MB | Colour scans, or uncompressed images | Downsample or re-scan |
| Over 5MB | Full-resolution scans at 600 DPI | Re-scan, or downsample hard |
A 40-page document at 24MB is 600KB per page — images. A 6-page document at 30MB is 5MB per page — scans, and there are enormous savings available.
This single number is more informative than any tool’s analysis, because it immediately tells you whether you are in the “a few percent available” range or the “90% available” range.
The three things that make PDFs large
Scanned pages — by far the most common
A scanner does not produce text. It produces a photograph of a page, and every pixel is stored.
The arithmetic is unforgiving. An A4 page scanned in colour at 600 DPI is roughly 4960 × 7016 pixels — about 35 million pixels, or 100MB uncompressed. Compression brings that down to a few megabytes per page, and that is after the scanner has done its best.
The same page scanned in greyscale at 200 DPI is about 1650 × 2340 pixels. That is roughly 3.9 million pixels — a ninth of the data, before compression.
This is why scanned PDFs dominate the “too large to email” category, and why they are also the easiest to fix dramatically. The detail is in how to compress a scanned PDF.
You can tell a scan at a glance: try to select text with your cursor. If nothing highlights, the page is an image.
Images at print resolution
A PDF exported for a printing press keeps images at 300 DPI or higher. Displayed on a screen at 96–150 DPI, everything above that is data you are carrying and nobody can see.
This is what happens when someone exports from Word or InDesign with “best for printing” or “press quality” selected. The fix is to re-export from the source with a screen-oriented preset, which is covered in how to reduce PDF file size.
One specific trap: cropping an image in Word or PowerPoint hides the cropped area but keeps storing it. A photo cropped to a tenth of its original frame still carries all of the original pixels into the PDF until you explicitly delete the cropped areas.
Embedded fonts
Every font family embedded in full adds roughly 100–500KB. On a normal document this is a rounding error. On a document assembled from many sources — a report combining sections from different authors, each using their own template — a dozen fonts can add several megabytes.
The fix is font subsetting, which stores only the characters actually used rather than the complete typeface. Most modern exporters do this by default; older ones and some LaTeX configurations do not.
Two cases that surprise people
Edit history you cannot see
Most PDF editors implement saving as incremental update: the new content is appended and the old content is marked superseded but left in the file. Ten rounds of edits can mean ten copies of the document inside one PDF.
This is also a privacy issue — text you deleted may still be recoverable from the file. If a document has been through several rounds of editing, a lossless rebuild both shrinks it and discards the superseded data.
Forms, attachments and thumbnails
A few less obvious contributors:
- Embedded file attachments. PDFs can carry arbitrary files inside them.
- Page thumbnails cached by older versions of Acrobat.
- Form field data and JavaScript in interactive forms.
- Layered content from design tools, where hidden layers are still stored.
None of these is usually the main cause, but together they explain why a document that should be small is not.
What to do with the answer
Once you know which case you have, the choice is straightforward:
Under 300KB per page — the file is already efficient. A lossless rebuild via our PDF compressor may find 5–30% by removing duplicate objects and edit history, and that is the ceiling. If you need it smaller than that, you need to remove content rather than compress it.
300KB–1MB per page — images at print resolution. Re-export from the source document if you still have it; that produces a better result than compressing the PDF afterwards. If you do not have the source, downsample.
Over 1MB per page — scans. Downsampling to 150 DPI, and greyscale if the original is a black-and-white document scanned in colour, typically cuts 80–95%.
Why the size matters at all
Almost everyone asking this question is trying to email something. The numbers worth knowing:
| Service | Attachment limit |
|---|---|
| Gmail | 25MB |
| Outlook.com | 20MB |
| Most corporate mail servers | 10–25MB |
| Safe for any recipient | Under 10MB |
Two catches. Attachments are encoded for transport, which inflates them by roughly a third — so a 20MB file can fail a 25MB limit. And the limit that actually matters is the recipient’s, which you cannot see; a message can leave Gmail happily and bounce at the other end.
Above 10MB, sharing a link is more reliable than fighting the file size.
What not to bother with
- Zipping the PDF. PDFs are already internally compressed. A zip saves a few percent and makes the file harder for the recipient to open.
- Deleting pages to hit a target when images are the problem. Fix the images and keep the document intact.
- Compressing the same PDF repeatedly. Each pass re-encodes already-degraded images. Two rounds look considerably worse than one for almost no extra saving.
- Uploading confidential documents to free online compressors. Contracts, payslips, medical letters and anything with a signature belong on your own machine. Our compressor runs in your browser for exactly this reason.