Why is my PDF so big? What fills it, and how to shrink it

A PDF is large because it stores everything it draws: text, fonts, pictures and any extra data attached to the pages. Most of the bulk is usually pictures, especially scanned pages and full-size phone photos. To shrink a file, find out what is big, then either redraw the pages at a lower resolution with Compress PDF or send only the pages you need. Compress PDF turns every page into a picture, so check that the result still works before you send it.

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What fills a PDF

• Scanned pages. Each scanned page is one full-page image, and its resolution sets its size. An A4 page at 300 DPI is 2480 × 3508 pixels, about 8.7 million. The same page at 150 DPI is 1240 × 1754, about 2.2 million. A mostly white scan still stores every pixel.
• Photos at camera resolution. A phone photo can be several thousand pixels wide. Placed small on a page, it still carries all of those pixels. Resize it before you put it in the document.
• Embedded fonts. The file carries the fonts it uses, so the page looks the same on any device. A complete font is larger than a subset holding only the letters the document uses. Most programs subset, but not all of them do.
• Repeated objects. A logo, photo or font can be stored again for each page, or for each file that was merged in. A merged file can keep several copies of the same image.
• Leftovers. Some editors save changes by appending them to the file, so older versions and deleted comments can remain inside it. Saving through a tool that writes a fresh copy usually drops them.

You can see which part is big. Split PDF in the "Every page as its own PDF" mode gives one file per page, packed into a ZIP. Open the ZIP and sort by size. If one or two pages are far larger than the rest, those pages hold most of the size, and they are usually scans or photos.

Try the simple fixes first

Some fixes need no compression at all. If the recipient only needs some pages, remove the rest with Delete PDF pages or Split PDF, and send that part. If you still control the source files, resize the photos before you place them in the document, using Resize image, so the page never stores more pixels than it shows. For a scanner, a lower resolution setting reduces the picture size in the same way, as long as the text still reads well at 100 percent zoom.

Compression is the right step when the pages are already final and you cannot go back to the source. It works on the whole file, so use it on the document you actually need to send, not on a folder of drafts.

How Compress PDF works

1. Drop the PDF onto the box. The status line counts through the pages as each one is redrawn.
2. Pick Light, Medium or Strong. Light keeps the most detail, and Strong gives the smallest file. Medium is the default.
3. Tick Black and white for text documents where colour does not matter. Each page is turned grey before it is saved.
4. Read the result. The tool shows how much smaller the file is. If the redrawn copy is not smaller, it says so and offers the original back instead of a bigger file.
5. Open the download and check small text at 100 percent zoom.

Behind the scenes, each page is drawn at the resolution you chose, saved as a JPEG, and placed on a page of the same size in a new PDF. Very large pages are capped so the browser does not run out of memory.

The trade-off: the text stops being text

Because every page is now a picture, the new file has no text to select, search, copy or fill in. Links and form fields stop working. Small print can blur at Strong. The original text layer is not carried over, so Ctrl+F finds nothing. That is the cost of the smaller file.

Compressing is the wrong choice when:

• You need to search, copy or quote the document.
• The document has form fields you need to fill in.
• Someone needs to read it with a screen reader, which cannot read text inside a picture.
• The file is an archive or evidence copy. Keep the original, and send the smaller copy only where size matters.
• The PDF is mostly text and vector drawings. It is already compact, and turning it into pictures can make it bigger.

Other ways to make it smaller

Option	Keeps the text?	Best when
Compress PDF	No	Scans and photo-heavy files you only need to read or send
Split PDF	Yes	You need only some pages, so you can leave the rest out
Export again from the program that made it	Yes	The source program has a smaller-file or minimum-size export option
PDF to JPG, then Images to PDF	No	You want to choose the resolution yourself

Splitting only helps when you drop pages, because the pages you keep stay the same size. If the file is too big for an email, the guide to attachment limits covers the options. To choose a resolution, read what the DPI setting does to your pages.

Follow-up questions

Can I get the text back after compressing?

Not from the compressed file, so keep the original. If you only have the compressed copy, you can render its pages and run OCR on them, but the result will be less accurate than the original text.

Why did my file come out bigger?

Compress PDF compares the sizes. If the original is already compact, a file of mostly text and vector drawings can grow when its pages are turned into pictures. In that case, keep the original.

Does compressing remove hidden text or document details?

The new file holds only the page pictures, so the original text layer and the document's properties are not carried over. That is a side effect, not a redaction method. Anything visible in a picture is still there.