Why Your PDF Is So Much Bigger Than the Images

Add up the source images before you blame the converter

Open the folder, select the images and look at the total. If it says 38 MB, then a 40 MB PDF is arithmetic rather than a fault. A PDF is a container: it holds each image as a compressed stream, plus a page description and a cross-reference table. On this site's converter that structure costs a couple of kilobytes for the whole document, not per page, and it is not something you can tune away.

This is also why the main converter on the home page does not quietly help you out — the tool above is the size-limit one, and it starts with a 200 KB limit already set. A baseline JPEG's own bytes are copied straight into the PDF, so the page is exactly as big as the JPEG was. That is deliberate, and it is the reason there is no quality loss, but it does mean nothing shrinks unless you ask for it.

Why a PDF gets so big after converting images

Three things drive the number, in rough order of how often they are to blame.

• Pixel count. A current phone shoots 12 megapixels or more, and a single photo off the camera roll is commonly a few megabytes. Twenty of those is a PDF you cannot email. Page settings do not change this: a 12 MP photo placed on an A6 page stores exactly the same pixels, it just prints them smaller.
• The wrong format for the content. PNG for photographs is the biggest avoidable cause, and it gets its own section below.
• Page count. Every image becomes a page and every page carries its own image stream. Scanning sixty sheets just in case gives you sixty times the problem.

What is not to blame: page size, orientation, margins and the resolution setting. Those decide how large the image is printed on the sheet. They do not change how many pixels are stored, so they barely move the file size at all.

PNG for photographs is where it really goes wrong

PNG compression is lossless, so it has to reproduce every pixel exactly. That is excellent for screenshots, logos, charts and line art, where large flat areas compress away to almost nothing. It is poor for photographs, where fine grain and sensor noise are close to random and there is very little to squeeze. The same photo saved as PNG is routinely several times the size of the JPEG.

There is a second surprise when a PNG goes into a PDF: the page can come out larger than the .png file on disk. PDF has no PNG filter, so the image is decoded and re-stored as an 8-bit RGB stream with lossless Flate compression. Pixel for pixel it is identical and nothing is degraded, but a greyscale scan or a small palette-indexed graphic is expanded to three full colour channels on the way, and transparency is written as a second stream alongside it. A small indexed PNG can grow a lot on that trip.

If your sources are photographs saved as PNG, re-saving them as JPEG first is the biggest single saving available. PNG or JPG for a PDF covers which format suits which kind of image.

A rough estimate you can do before converting

Start from the raw pixel data, which is width times height times three bytes. A 4000 by 3000 photo is 36 MB of raw pixels. Then apply a rough compression ratio.

Add a couple of kilobytes for the container and that is your answer. The ratios move with the content, but they are close enough to tell you whether you have a compression problem or simply too many photographs.

The fixes, in order of what you lose

Work down this list and stop as soon as you are under the limit. Each step costs more than the one above it.

1. Remove pages you do not need. Free. Delete the thumbnails you are not sending before you convert.
2. Re-save photographic PNGs as JPEG. Costs one generation of JPEG compression, which at high quality you will not see, and often cuts the page by three quarters.
3. Reduce the pixel dimensions. A full A4 page at 150 dpi needs about 1240 by 1754 pixels, and anything beyond that only helps someone who will zoom in or print. This converter has no resize control, so do it first in Preview, Photos, Paint or IrfanView.
4. Lower the JPEG quality. On the main converter on the home page, open More options and set Image quality to Smaller file. Everything is then re-encoded as JPEG at around 82 per cent, and any transparency is flattened onto white.
5. Set a hard byte target, which is the next section.

One thing this site cannot do is compress a PDF you already have. It builds PDFs from images and will not take a PDF as input. If the originals are gone, macOS Preview can shrink one through File, Export, Quartz Filter, Reduce File Size, though it is blunt and the result often looks it. Acrobat's compression gives you more control if you have it.

Setting a hard byte limit

The converter above arrives with a 200 KB limit already selected. The menu runs from 50 KB to 5 MB, a Custom box takes anything from 5 KB to 50 MB, and No limit — best quality switches the whole thing off. It builds the PDF at JPEG quality 92 first, which is the best this mode will do — even that first pass re-encodes, because asking for a byte ceiling means giving up the untouched-bytes route. If it does not fit, the tool searches downwards through JPEG quality settings, and only when quality alone cannot get there does it begin reducing pixel dimensions, ten per cent at a time. It measures the assembled PDF on every pass rather than the image, because the container costs bytes too, then shows you the byte count it actually achieved.

Two things worth saying plainly. This mode discards image data on purpose, and so does Smaller file under Image quality. Only JPG and PNG ever go in untouched anyway: HEIC, TIFF, WebP, AVIF, GIF, BMP and SVG have no place in a PDF, so they are decoded and re-encoded once whatever you choose. And there is a floor, because a PDF carries a couple of kilobytes of structure whatever the image quality, so a very small target on a real photograph can be unreachable. When that happens the tool reports the smallest file it managed rather than pretending it hit the number. The same tool has its own page at image to PDF under a size limit.

Sometimes the file is right and the limit is wrong

If the PDF is going to a print shop, an archive or a legal record, the size is the point, and compressing a 300 dpi scan to clear a mail limit ruins what you made it for. Image to PDF at 300 dpi covers the print side.

It is also worth checking what the limit actually is before you start destroying quality. The bounce message usually names a figure, and the receiving server can be stricter than the one you send from, so the smaller of the two applies. Two fixes that cost no quality at all: split the images across two or three smaller PDFs, or put the full-size file somewhere and send a link instead.