How to compress a file to a specific size (PDF, image, video)

To compress a file to a specific size, use the method that suits its type: lower the image quality and resolution inside a PDF, lower the quality or pixel size of a photo, and lower the bitrate, resolution or length of a video. Zipping only helps with files that aren't compressed already, such as documents, spreadsheets and text; photos, videos and most PDFs hardly shrink in a ZIP.

First, pin down the target

Check the exact limit and its unit. "200 KB" can mean 200,000 or 204,800 bytes, depending on who built the form; our guide to KB vs MB explains how to pass either way. For email, Gmail's limit for personal accounts is 25 MB, and bigger attachments are swapped for a Google Drive link (Gmail Help). Work and school accounts, and other email services, set their own limits, so check their help pages. Always aim a little below the limit.

Which method suits which file

How to compress a PDF

A scanned PDF is a set of page photos, so its size depends on the resolution and compression of those images. A PDF exported from Word or Google Docs is usually small already; if it is large, it probably contains big images or embedded fonts.

• In your browser: our Compress PDF to size tool takes a target such as 100 KB, 200 KB or 500 KB and finds the highest quality that fits. It redraws each page as an image, so the text can no longer be selected; keep your original. Nothing is uploaded.
• On a Mac: in Preview, choose File > Export, then pick Reduce File Size in the Quartz Filter menu. Apple notes the result may be lower quality and that the size varies (Apple).
• On iPhone (iOS 26 and later): the Preview app can export a PDF with Optimize Images for Screen and Save Images as JPEG turned on, both of which make smaller files (Apple).
• With Adobe Acrobat Pro (paid): Save as Other > Optimized PDF lets you downsample images, unembed or subset fonts, and discard extras such as thumbnails and comments (Adobe).

If a PDF still won't fit, remove pages the form doesn't need, or rescan in black and white or greyscale when colour isn't required.

How to compress an image

JPEG is a lossy format: it saves space by discarding detail you mostly don't notice, and the quality setting controls how much (JPEG committee). PNG is lossless (W3C), so it has no quality setting to lower. For photos and screenshots that must be small, save them as JPG.

You have two levers: quality and pixel count. Halving the width and height leaves a quarter of the pixels. A 4000 × 3000 photo has 12 million pixels; at 2000 × 1500 it has 3 million, which can reach a far smaller file at the same quality.

• In your browser: the Image compressor lowers JPG or WebP quality step by step until the photo fits a target in KB. If it can't get there, shrink the dimensions first with the Image resizer. If a form also sets a minimum, such as 20–50 KB, Increase image size can bring a small file into that range.
• On a Mac: in Finder, Control-click the image, choose Quick Actions, then Convert Image, where you can pick a format and a smaller size (Apple).
• On iPhone (iOS 26 and later): in Preview, export the image as JPEG and drag the Size slider left for a smaller file (Apple).

How to compress a video

A video's size is roughly its bitrate multiplied by its length. That gives you three ways to shrink it: lower the bitrate, lower the resolution so fewer pixels share that bitrate, or cut the length.

To hit a target, work the bitrate out backwards. For a 3-minute clip (180 seconds) that must stay under 25 MB:

1. 25 MB is 200,000 kilobits (25 × 8 × 1,000).
2. Divided by 180 seconds, that is about 1,111 kilobits per second (kbps) in total.
3. Take off 128 kbps for the audio, leaving about 983 kbps.
4. Set the video to around 880 kbps, 10% lower, to leave room for the file's overhead.

The free, open-source video converter HandBrake can encode to an average bitrate. Its documentation explains the trade-off: an average bitrate controls the output size but not the quality, while its default constant-quality mode gives sizes you can't predict (HandBrake).

Built-in options:

• On a Mac: in QuickTime Player, choose File > Export As and pick 720p or 480p. For 1080p or 4K, choose Smaller File Size to export in HEVC (Apple).
• On Windows: the Clipchamp editor exports at 480p, 720p, 1080p or 4K (1080p is the largest free option), and Microsoft describes 480p and 720p as the choices for smaller files (Microsoft).

If the video is still too large, trim it, or share a cloud link instead of an attachment.

When a ZIP file helps

A ZIP file is lossless: everything comes back exactly as it was. It works well for documents, spreadsheets, text and code, and for bundling many files into one attachment.

• Windows: right-click the file or folder, select Show more options > Send to > Compressed (zipped) folder (Microsoft).
• Mac: Control-click it and choose Compress (Apple).

If someone sends you a ZIP and you're on a phone or a shared computer, the ZIP extractor opens it in your browser.

Why some files won't shrink

• They are already compressed. Microsoft notes that JPEG images are already highly compressed, so a ZIP of JPEG photos is about the same size as the photos (Microsoft). The same goes for MP4 videos, MP3 audio and most PDFs full of photos: zipping them finds almost nothing left to remove.
• The target is too small for the content. A tight limit on a long scanned document leaves only a few KB per page, and below some point the text stops being readable. When a tool says it can't reach the target, change something else: fewer pages, fewer pixels, black and white, or a shorter clip.
• Lossless formats have a floor. A PNG or ZIP can only remove repetition; it can't throw detail away.
• The file has been compressed before. Each lossy save loses a little more quality, so always start again from the original rather than squeezing an already-compressed copy.

Common mistakes

• Compressing your only copy. Keep the original, especially for PDFs whose text becomes an image.
• Zipping photos to meet a size limit. Lower the quality or the pixel size instead.
• Ignoring KB versus KiB. Leave a margin below any limit.
• Uploading at full camera resolution. Many forms state the pixel size they want; resize to that first, then compress.