Why scans are so big

When you scan a letter or a form with your phone, each page is saved as a photograph of the paper. A typed document exported from a word processor stores its text as text, which takes very little space. A scan stores the same words as millions of pixels. Multiply that by several pages and a short scan can easily come to several megabytes.

That is often too large to send comfortably. Many email services cap attachments at around 20–25 MB for the whole message, some workplace mail servers set lower limits, and upload forms for job applications, claims or government services often accept only a few megabytes per file.

First, check Apple's built-in options

Mail Drop. If a message is too large for your email provider, the Mail app on iPhone can offer to send the attachment with Mail Drop instead. The file is uploaded to iCloud and the recipient gets a link to download it, valid for 30 days. Apple explains the details and limits in Mail Drop limits. This is enough when you only need the recipient to get the file and you don't mind it going through iCloud. It does not make the PDF smaller, so it won't help with an upload form that has a size limit.

Preview. On iOS 26 and later, Apple's Preview app can export a PDF with options to reduce image resolution for screen viewing and to save images as JPEG, which makes a smaller copy on the device. See Export or compress PDFs and images in Preview on iPhone. Try this first. If the result is small enough and still readable, you are done.

When a dedicated compressor helps

Apple lists these PDF options as switches you turn on or off, without a target size or resolution setting. With a scan you usually want more control: the smallest file that still leaves the small print legible, or a file just under a specific limit. That means choosing the image resolution and quality, and checking the result before you send it.

Smart PDF Compressor is a free, offline PDF compressor for iPhone and iPad that helps anyone emailing, uploading or storing a PDF shrink it by up to 95%, without the file ever leaving the device. It is made by Quan Dang, with no account and no subscription.

  • It detects that a PDF is a scan and picks settings to suit it.
  • Light, Standard and Maximum presets cover most cases. For finer control you can set image quality, resolution from 72 to 300 DPI, and grayscale.
  • A side-by-side preview lets you compare the original and the compressed file before saving, so you can check that the text is still sharp enough.
  • Several scans can be compressed in one batch.
  • Everything happens on the device, with no internet connection needed. Nothing is uploaded and no data is collected.

Settings that tend to work for scans

  • Start with Standard. Move to Maximum only if the file is still too large.
  • Use grayscale for black-and-white paperwork. Colour adds size and rarely helps a typed letter or a form.
  • Lower the resolution before the quality. Text on a scan usually stays readable on screen at modest resolutions. Keep it higher if the recipient will print the document or it has very small print.
  • Always check the preview. Signatures, stamps and numbers are the first things to become hard to read.