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Compress a Scanned PDF for Upload Size Limits

Updated October 2026

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Scanned PDFs are big because every page is a photo; shrink them with Preview's Reduce File Size on Mac or Preview's export options on iPhone, scan fewer and simpler pages, and remove any password before uploading.

Visa, passport, exam and loan portals often cap uploads at a few hundred kilobytes or a couple of megabytes per file. Scanned PDFs blow past that because each page is stored as a photo. The quickest fixes are to re-save the PDF with image compression (Preview on Mac or iPhone does this for free), scan only the pages you need, and remove any password first, because many portals can't read protected files.

Before you compress

  • Check the exact limit and format on the portal page: the maximum size, whether it's per file or in total, and whether it wants PDF or JPEG. Rules differ between portals and change, so go by what the upload page says.
  • Remove the password. A portal can't open a protected PDF to check it. See iPhone, Mac, Windows or Android.
  • Keep the original. Compression lowers image quality. Keep a full-quality copy in case the portal rejects a blurry scan.

On a Mac: Preview

  1. Open the PDF in Preview and choose File > Export.
  2. In the Quartz Filter menu, choose Reduce File Size.
  3. Give it a new name and click Save.

Apple notes the compressed PDF “may be of lower quality than the original”. The same Export window also offers Optimize images for screen and Save images as JPEG (Apple: Reduce the size of a PDF in Preview on Mac). Reduce File Size can make text-heavy scans hard to read, so open the result and zoom in before uploading.

On iPhone or iPad: Preview (iOS 26 and later)

  1. Open the PDF in the Preview app.
  2. Tap the menu next to the filename, then tap Export.
  3. Choose PDF, then turn on Optimize Images for Screen and Save Images as JPEG.
  4. Tap the confirm button, choose a location, rename it and tap Save.

Apple says Optimize Images for Screen “creates a smaller file” (Apple: Export or compress PDFs in Preview on iPhone). If the portal wants a JPEG instead, the same screen exports to JPEG with a size slider.

Scan smaller in the first place

  • Only the pages asked for. Delete extra pages before uploading. Preview on iPhone can delete and reorder pages (Apple).
  • Use the document scanner, not the camera. Notes, Files and Preview crop the page and flatten it (Apple: How to scan documents on iPhone). A cropped scan is smaller than a photo of a desk.
  • Greyscale or black and white for text documents, if the portal allows it. Preview's scanner lets you adjust the colouring of each scan (Apple). Keep colour for photos and anything with stamps or seals the officer must see.
  • One document per file. If the portal has separate slots, don't merge everything into one huge PDF.

Common problems

  • Still too big: export again with both options on, or scan again in greyscale.
  • Text is blurry: you've compressed too much. Go back to the original and use only one of the options.
  • Portal says “file is corrupt” or “encrypted”: the file is probably still password-protected or restricted. Remove the password and try again.
  • File made bigger: PDFs that are mostly real text are already small. Re-saving can add bulk. Upload the original.

With UnKey

UnKey Pro on iPhone, iPad and Mac includes a Compress tool aimed at scans. It re-encodes photo-like pages and leaves pages with real text untouched, so the text stays sharp, and it never returns a larger file than you gave it. It works offline, so your passport or bank scan isn't uploaded to a compression website. Unlocking is free; Compress is part of the one-time Pro purchase.

Questions

Will compressing make my documents unreadable?

It can if you overdo it. Always open the compressed copy, zoom in on small print, and keep the original.

Why is a 2-page scan 8 MB?

Each scanned page is a high-resolution photo. Text PDFs from a bank are tiny because they store text, not pictures.

Do portals accept password-protected PDFs?

Many can't read them. Remove the password before uploading unless the portal says otherwise.

Are online compressors safe for passports and statements?

They upload your file to a server. For identity and financial documents, compressing on your own device is the safer choice.

Sources

Formats can change. If a password doesn't work, check the email the document came with or contact the issuer. UnKey isn't affiliated with any bank, employer or authority named here.

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