How to Make a PDF Smaller (and Why Some PDFs Won't Shrink)

Updated 27 September 2026 · 6 min read

A PDF is only as big as what's inside it. Most oversized PDFs are full of photos or scanned pages, and the only way to shrink those properly is to make the images smaller. Here's what takes up the space, what a free in-browser compressor can and can't do, and the fixes that make the biggest difference.

What makes a PDF big

Almost every large PDF owes its size to one of three things.

• Images. Photos pasted into a report, a logo saved at poster size, a screenshot dropped in as PNG. A PDF usually keeps each picture at its original resolution, even when it's shown small, so one phone photo can add several megabytes.
• Scanned pages. A scan isn't text, it's a photo of a page. An A4 sheet scanned at 300 dpi is about 2,480 × 3,508 pixels, close to 8.7 million pixels, and a 20-page scan stores that 20 times.
• Embedded fonts. PDFs carry their fonts so the document looks the same everywhere. Usually only the characters used are included, but fonts with large character sets, such as Arabic, Chinese or Japanese, can add noticeably more when embedded in full.

Then there's structural overhead: the internal index, duplicated resources, and old copies of objects left behind by editing. Some programs save changes by adding them to the end of the file, so a form that has been filled in and re-saved several times can carry data it no longer uses.

To see which kind you have, try to select a single word. If you can highlight text, the page contains real text; if you can't, it's an image. Also divide the file size by the number of pages: a text-only document rarely needs more than a few dozen KB per page, so if yours is nearer a megabyte, images are the reason.

Why some PDFs won't shrink

Photos and scans inside a PDF are almost always stored already compressed, usually as JPEG. Squeezing that data a second time without lowering quality gains next to nothing. To make an image-heavy PDF much smaller, something has to reduce the images themselves: fewer pixels, lower JPEG quality, or both. That always costs some detail.

That's the honest limit of our Compress PDF tool. It removes structural overhead but deliberately does not recompress images, so it never makes your pages blurrier. The flip side is that a scanned or photo-heavy PDF may shrink only slightly, or not at all, and so may a PDF that was already saved efficiently.

How to shrink a PDF with SAA Tool

1. Open Compress PDF and drop your file on the box, or click Browse.
2. Click Compress / Optimize. The tool copies every page into a fresh file and saves it with compressed object streams, which drops unused leftovers and packs the internal structure more tightly.
3. The new file downloads as SAA-Tool-Compressed.pdf, and the page shows the size before and after in bytes.

Everything happens in your browser, so the file never leaves your device. Text-based documents and PDFs that have been edited and re-saved many times tend to benefit most. Because the tool builds a new file, bookmarks, fillable form fields and document details such as the title may not carry over, so check the result and keep your original. Password-protected PDFs need the password removed first.

If the number barely moves, your file is big because of images, and the next sections are where the real savings are.

Fix the source: smaller images before the PDF is made

• Resize photos first. A photo 4,000 pixels wide shown across half an A4 page doesn't need to be more than about 1,500 pixels wide to look sharp on screen. Going from 4,000 × 3,000 to 1,500 × 1,125 cuts the pixel count to roughly a seventh. Use the Image Resizer, then the Image Compressor, before inserting them. Our guide to resizing and compressing images has target sizes.
• Compress pictures in Word. Select a picture, go to Picture Format > Compress Pictures, untick Apply only to this picture so it covers the whole document, tick Delete cropped areas of pictures, and choose Web or E-mail under Resolution. Microsoft explains the options on its picture compression help page.
• Save as PDF with the smaller setting. In Word for Windows, choose File > Save As, pick PDF as the file type, and select Minimum size (publishing online) next to Optimize for. Microsoft's save as PDF instructions cover the other Office apps too.
• On a Mac, open the PDF in Preview, choose File > Export, and pick Reduce File Size from the Quartz Filter menu. It recompresses images hard, so check small text afterwards. Apple describes it in the Preview user guide.

Scanning settings that save the most space

• Resolution. 150 to 200 dpi is plenty for reading on screen and emailing. 300 dpi is worth it when the document will be printed or needs text recognition. An A4 page at 150 dpi is about 1,240 × 1,754 pixels, a quarter of the pixels of the same page at 300 dpi.
• Colour mode. Use black and white (sometimes called text mode) for plain typed pages, greyscale for pages with stamps or signatures that must stay legible, and colour only when colour matters, such as an ID card. A colour image stores three values per pixel; greyscale stores one.
• Crop to the paper. Scanning a receipt on the full A4 glass stores a lot of empty white.

A worked example

Say an upload form accepts PDFs up to 2 MB and your six-page scanned document is 9 MB.

1. Run it through Compress PDF first. It takes seconds and loses nothing. If the result is still around 9 MB, the scanned images are the cause.
2. If you can rescan, use 150 dpi greyscale. That's a quarter of the pixels of 300 dpi and one value per pixel instead of three, which often brings a file like this under the limit.
3. If you can't rescan, convert the pages to JPG with PDF to Images (Scale 2 keeps small print readable), shrink them with the Image Compressor, then combine them into a PDF again. On Windows, select the images in File Explorer, right-click, choose Print (on Windows 11 it may be under Show more options) and pick Microsoft Print to PDF. On a Mac, open them in Preview, select all the thumbnails and use File > Print > PDF > Save as PDF.

If the document can be sent in parts instead, splitting the PDF by page range is another option.

Frequently asked questions

Does compressing a PDF reduce its quality?

With our Compress PDF tool, no. It doesn't touch text or images; it only rewrites the file's structure. Tools that lower image resolution or JPEG quality do reduce quality, which is how they get bigger savings.

Why did my PDF stay the same size, or get bigger?

The original was probably already saved efficiently, and rewriting it can even add a few bytes. The tool shows both sizes, so if the new file isn't smaller, keep your original.

Is my PDF uploaded anywhere?

No. The file is read and rewritten inside your browser and saved straight back to your device.

How small does a PDF need to be for email?

Gmail's attachment limit is 25 MB, according to Google's help page, and Outlook for desktop has a default limit of 20 MB for internet email accounts, according to Microsoft. Company mail often has lower limits, and limits change, so staying under 10 MB is a safe habit. For anything bigger, share a cloud link.