Why PDFs get so big, and how to shrink them

It's almost always the images

A PDF contains text, lines, images and fonts. Text and lines are stored as short instructions ("draw these letters here"), so even hundreds of pages of them add very little. Photos and scans, on the other hand, store the color of every pixel, and a single one can outweigh hundreds of pages of text.

So most size problems come down to one question: which images are in this PDF, and at what resolution? That's why a report saved straight from a word processor is light, and a stack of receipts from a phone scanning app is heavy.

A few other things can add weight too:

• Embedded fonts: fonts are packed into the file so it looks the same on any computer. Fonts for languages with large character sets are bigger, but usually only the characters actually used are included, so fonts rarely rival images.
• Edit history: some PDF editors save each change by appending it to the end of the file, so a document edited many times can grow out of proportion to its content. "Save As" often cleans this up.
• Hidden content: attachments, layers and form data can be in the file without being obvious on screen.

There are exceptions. PDFs packed with hundreds of thousands of lines and points, such as engineering drawings, detailed maps or complex charts, can be heavy without a single photo, and slow to display as well. For these, turning each page into one picture can actually make the file lighter than storing every line, so if nobody needs to zoom in closely, the recompression method below is one of the rare cases where it fits well.

First, figure out why your PDF is heavy

Guessing the cause before picking a method saves a lot of trial and error. Three checks are usually enough.

1. Work out the size per page: divide the file size by the page count. Tens of kilobytes per page means a mostly-text document; over 1 MB per page almost certainly means large images.
2. Try selecting text. Drag across a line in your PDF viewer. If individual characters highlight, it's a text PDF. If the whole page selects as one picture, or nothing selects, it's a scanned (image) PDF.
3. Find the heavy pages. In a long document, a few scanned attachments at the end often account for most of the size. Flip through and note which pages are photos or scans.

Method 1: export again from the original file

If you still have the original Word, PowerPoint or similar file, the cleanest option is to export the PDF again. The text stays real text, only the photos get smaller, and search and copy keep working.

Most document apps offer image-quality, "minimum size" or "for web" choices when saving or exporting to PDF, and many can also compress the pictures inside the document beforehand. Menu names differ by app and version, so start by opening the Options button in the save dialog.

Simply not pasting full-size phone photos (thousands of pixels wide) into documents makes a big difference. For a photo that appears palm-sized on the page, 1,000–1,500 pixels on the long side is hard to tell apart on screen or in print.

Method 2: change your scan settings

In a scanned PDF every page is a picture, so resolution and color mode at scan time largely decide the size. If you can still rescan, this is the most effective place to make a change.

• Resolution: doubling the resolution doubles both width and height in pixels, so the pixel count goes up four times. An A4 page at 300 dpi is about 2480 × 3508 pixels; at 150 dpi it's about 1240 × 1754, a quarter of the pixels. For ordinary documents read on screen or submitted for office use, the lower setting is often enough.
• Color: color scans store three color values per pixel. Unless color matters, as with stamps or colored charts, scan in grayscale. Plain text pages are smallest in black-and-white (1-bit) mode, but faint text and stamps can disappear.
• Format: if your scanner software offers something like "compressed PDF" or "high-compression PDF", it usually saves smaller files than plain PDF.

Method 3: recompress an existing PDF as images

If you have neither the original nor the chance to rescan, the remaining option is to redraw each page as a lower-resolution image and build a new PDF from them. ToolyStation's Compress PDF works this way, entirely inside your browser.

This suits scanned PDFs, which are images already: you're just swapping one picture for a smaller one, so little is lost. On a text PDF, though, the text becomes part of a picture, search, copy and links stop working, and the file can even get bigger.

Pick the quality by asking whether the smallest print has to stay readable. For documents where footnotes or numbers in tables matter, start with High. For photo-heavy material or large type, Medium or lower is often fine.

Method 4: drop pages you don't need, or split the file

The surest way to a smaller file is less content. Look for appendices nobody asked for, duplicate pages of the same evidence, and blank pages. Delete them in Merge & Arrange PDF and save, and the new file contains only what's left.

If the recipient accepts several uploads, splitting the main document from the attachments is another option. When there's a per-file limit but no overall limit, splitting preserves quality better than forcing everything down to a low setting.

Example: an application that must be under 10 MB

Say you need a resume (Word), a cover letter (Hancom or another word processor) and three scanned documents (a diploma and two certificates) combined into one PDF under 10 MB.

1. Save the resume and cover letter as PDFs from their own apps. As text PDFs, each typically comes out at a few hundred kilobytes or less.
2. Rescan the three documents in grayscale, or, if you already have scanned PDFs, run just those through Compress PDF at High first.
3. If the documents are phone photos, turn them into a PDF with Images to PDF. It resizes images to 1,500 pixels on the long side, so the result is far lighter than the original photos.
4. Combine them in Merge & Arrange PDF in the order resume, cover letter, documents.
5. If the result is over 10 MB, go back a step and compress only the scanned documents one level lower. Leaving the resume and cover letter uncompressed keeps them searchable.

The key is to shrink only the heavy parts and leave the light text documents alone. Compressing everything at once is quicker, but a recruiter who wants to search for your name or school won't be able to if the text has turned into a picture.

Common mistakes when shrinking PDFs

• Compressing an already compressed file: lossy formats like JPEG lose a little quality every time they're saved. Recompressing a compressed PDF with another tool stacks the losses and quickly smears the edges of text, so always start from the original when you try a different setting.
• Screenshotting pages and turning them back into a PDF: screenshots are captured at screen resolution, which is too low for printing; the text becomes a picture; and page sizes end up inconsistent. It's quick in a pinch, but the result is usually the worst of all the options.
• Expecting a ZIP to make it smaller: the images and text inside a PDF are usually stored compressed already, so zipping the file barely changes its size. It doesn't help with upload limits and only makes the recipient unzip it.
• Lowering resolution all the way just to hit a number: even when the goal is a size limit, if the information the recipient needs gets smudged you may have to resubmit. Stop when you're a little under the limit and spend any headroom on quality.

What to check after shrinking

• The smallest print: zoom to 100% or more and make sure footnotes, numbers in tables and ID or account number fields are readable.
• Stamps and signatures: if you converted to grayscale, check that a stamp is still recognizable as an original. Keep color for documents where it matters.
• Page count and order: check the last page number to be sure nothing was lost while merging and deleting.
• Text search: if parts should stay text PDFs, confirm that searching finds words in them.
• Headroom: some systems count a megabyte as 1,000,000 bytes and others as 1,048,576 (1024 × 1024), so a file your computer shows as 9.9 MB might be treated as over a 10 MB limit elsewhere. Leave 5–10% of headroom rather than aiming exactly at the limit.
• File name: rename the file if the recipient has a naming rule.

Finally, always keep the originals. A PDF recompressed as images can't be restored to its original quality, so if you ever need better quality you'll have to start again from the original.