How to Compress a PDF on iPhone

To compress a PDF on iPhone, open the file in a PDF tool that works on the device, choose a compression level and save the smaller copy. Most large PDFs contain large images, so the main job is to reduce the image size and keep the text readable. Keep the original file before you compress it, because compression removes some detail and cannot always be undone. The steps below explain why PDFs get large and how to choose the right setting.

Why PDFs get large

A PDF stores the pages, the text, the fonts and the images. The size depends mostly on the images. Three things make a file large:

• High-resolution images. A photo taken by an iPhone camera has many more pixels than a page needs.
• Scanned pages. A scanner stores each page as an image. A scan at high resolution can be large, even when the page has little text.
• Embedded fonts and extra data. Fonts and objects add some size, but images are usually the largest part.

A PDF made from photos is often larger than a PDF made from text, because the photos hold far more data.

Image quality and file size

Image resolution is measured in dots per inch (dpi). A higher dpi gives more detail and a larger file. The number of pixels grows with the square of the dpi. This is simple math:

Check the arithmetic: 150 × 150 = 22,500, and 300 × 300 = 90,000. The second is four times the first. So a page scanned at 300 dpi holds four times as many pixels as the same page at 150 dpi.

For reading text on a screen or on paper, 150 to 200 dpi is often enough. For a photo that you want to print large, keep a higher setting. Choose the lowest setting that keeps the text clear.

Step by step

1. Keep a copy of the original PDF. Save it in a folder you can find again.
2. Open the PDF in your PDF tool.
3. Choose the compression tool, and select the file.
4. Pick a level. Start with a moderate level, and keep the text readable.
5. Preview the result. Zoom in on small text, tables and images.
6. Save the new file with a clear name, such as "contract-small" or "receipt-2026-11".
7. Check the size, then send or upload the smaller copy.

If the text looks blurry, go back to the original and choose a higher level. Do not compress the same file many times, because each pass removes more detail.

Choose the right level

Use this table as a guide for common uses. The levels in your app may have other names.

A scanned page with a signature needs extra care. Zoom in on the signature after compression. If the strokes look broken, choose a lighter level.

Image compression and PDF compression

Two different actions can make a PDF smaller.

• Image compression reduces the size of the photos inside the file. This often gives the biggest saving.
• PDF compression reduces the whole document. It can remove duplicate objects and clean up the file structure.

Use image compression first, then check the result. Use PDF compression when you need a smaller file and the images are already small.

Check before you send

Before you send the smaller file, check these points:

• Text. Zoom in to 100 percent and read a small line.
• Images. Check a photo or a logo, and look for blur or blocks.
• Pages. Check the page count and the order.
• Forms. Check that fields and checkboxes still work, if the file is a form.
• Signatures. Check that the signature still looks like a signature.

Keep the original file, and send only the copy you have checked.

Privacy while you compress

A PDF often contains names, addresses, bank details or medical information. A compression tool that works on the device keeps the file on your iPhone during the task. A cloud tool sends the file to a server, which makes an extra copy that you do not control.

Calm PDF processes every document on your device. Compression runs locally, no file is uploaded and no account is needed. The app's on-device text recognition also keeps the page image and the recognized text on your iPhone.

How Calm PDF does it

Calm PDF is a PDF and image toolkit for iPhone. The free version includes image compression, which reduces the size of photos. The PDF compression tool, which reduces the size of the whole document, is part of the Pro upgrade. The free version also includes image-to-PDF conversion, PDF merging and PDF to JPG or PNG export. The Pro upgrade also adds HEIC and HEIF conversion and OCR text extraction. The full list is on the Calm PDF app page.

Calm PDF works in one flow: pick the file, choose the compression, preview and save. Nothing is uploaded, and no account is created. The app has no tracking and no analytics.

For a related task, read how to convert photos to PDF on iPhone. If you need to combine files first, read how to merge PDF files on iPhone.

If you want to make a PDF smaller on your iPhone, try Calm PDF. You can find more tools on the apps page.

Frequently asked questions

Does compressing a PDF reduce its quality?

Yes, some detail is removed. Light compression keeps most of the quality, and strong compression can make text and images look soft. Check the result before you send it.

Why is my scanned PDF so large?

Scans store each page as an image, often at a high resolution. Reduce the image resolution or use compression to make the file smaller.

Should I compress a PDF before I merge it?

It depends on the job. Merging first makes one file to compress. Compressing first can help when one of the files is very large. Keep the original files in either case.

Can I compress a PDF without an internet connection?

Yes, if the tool works on the device. Calm PDF processes files on your iPhone, so the compression does not need an internet connection.