How to Compress PDF File Size

Why PDFs Get So Large

PDF is a container format. The file size is almost entirely determined by what is packed inside:

• Embedded images are the usual culprit. A single uncompressed 12-megapixel photo can add 30+ MB. Scanned documents are full-page images, so even a five-page scan can weigh 80 MB.
• Embedded fonts. A PDF that includes the full font file instead of a subset adds a few hundred KB per font — minor on its own, but it adds up.
• Duplicate objects. PDFs edited multiple times can accumulate unused objects that inflate the file without contributing visible content.

Knowing the source of the bloat tells you which compression method will actually help. If images are the problem, re-encoding them at a lower quality is the biggest lever. If the PDF is mostly text, stripping metadata and subsetting fonts is more effective.

Method 1: Create a Smaller PDF from Images in the First Place

The most effective compression happens before the PDF exists. If you are converting photos or scans into a PDF, the converter you choose — and its quality setting — controls the output size directly.

The JPGtoPDF converter has a Smaller File mode that re-encodes each image at JPEG quality 72 during conversion. This typically cuts the output PDF by 40–60 % compared to the Original quality setting, with no visible difference in most photos. Because the tool runs entirely in your browser, nothing is uploaded to a server — your files stay private.

How to do it

1. Open jpg-to-pdf.app and drop your images onto the page.
2. In the settings panel, set Photo quality to Smaller file.
3. Click Convert to PDF. The resulting file will be significantly smaller than an original-quality conversion.

Method 2: macOS Preview “Reduce File Size” Filter

If you already have a PDF and you are on a Mac, Preview can compress it with no extra software.

1. Open the PDF in Preview (double-click it, or right-click → Open With → Preview).
2. Go to File → Export….
3. In the Quartz Filter dropdown, select Reduce File Size.
4. Choose a save location and click Save.

This can dramatically shrink image-heavy PDFs — often by 70 % or more. The trade-off is that Preview uses aggressive downsampling (roughly 72 DPI), so images may look blurry when zoomed in. For PDFs you plan to view only on screen, this is usually fine. For print, keep a high-quality copy as well.

Method 3: Adobe Acrobat “Reduce File Size”

Acrobat Pro (and Acrobat Reader DC to a limited extent) can optimize PDFs while preserving more quality than Preview.

1. Open the PDF in Acrobat.
2. Go to File → Save As Other → Reduced Size PDF (or File → Compress a PDF in newer versions).
3. Pick a compatibility level — Acrobat 10 or later gives the best compression while keeping broad compatibility.
4. Save the file.

For more control, use File → Save As Other → Optimized PDF, which lets you choose image resolution targets, font handling, and whether to discard metadata, bookmarks, or form fields. This is the most granular option, but it requires a paid Acrobat Pro license.

Method 4: Free Online PDF Compressors

If you need to shrink an existing PDF and do not have Acrobat, several free web tools can do it:

• iLovePDF — lets you choose between low, medium, and high compression.
• Smallpdf — one-click compression with a preview of the result.
• PDF24 — offers granular DPI and quality controls.

The main concern with online compressors is privacy: your file is uploaded to a third-party server. Most services delete files within an hour, but if your PDF contains sensitive data (tax documents, medical records, contracts), a local method is safer.

Method 5: Ghostscript (Command Line)

For batch processing or automation, Ghostscript is a free, open-source tool that runs on Windows, macOS, and Linux. It gives you fine-grained control over compression.

The -dPDFSETTINGS flag controls the aggressiveness:

• /screen — 72 DPI, smallest file, low quality.
• /ebook — 150 DPI, good balance for on-screen reading.
• /printer — 300 DPI, suitable for printing.
• /prepress — highest quality, minimal compression.

Install Ghostscript via brew install ghostscript on macOS, sudo apt install ghostscript on Ubuntu/Debian, or download from the Ghostscript website on Windows.

Comparison: Which Method Should You Use?

Tips for Keeping PDFs Small

• Resize images before converting. A 4000 × 3000 photo scaled to 2000 × 1500 is one-quarter the pixel count and roughly one-quarter the file weight. If the PDF will only be viewed on screen, you rarely need more than 150 DPI (1240 × 1754 pixels for A4).
• Use JPEG for photos, PNG for graphics. JPEG compresses photographic content far better than PNG. Save PNG for screenshots, diagrams, or anything with sharp text and flat colors.
• Avoid scanning at unnecessarily high DPI. 300 DPI is plenty for printed text. 600 DPI doubles the file size with minimal readability gain for most documents.
• Strip metadata you do not need. GPS coordinates, camera model, edit history — these add weight. Ghostscript and Acrobat can strip them during compression.
• Split very large PDFs into smaller ones. If a 200-page scan is too large even after compression, split it into logical chapters. Most PDF viewers and online tools support page-range extraction.

Common Upload and Attachment Limits

Knowing the target size helps you pick the right compression level:

For the tightest limits (5–10 MB), combine aggressive image compression with a lower page count. If you are building the PDF from scratch, the converter's Smaller File mode is the easiest way to hit these targets without a multi-step workflow.

Related

• Combine Multiple JPGs into One PDF

  Merge a batch of photos into a single document.

• PNG to PDF

  Convert screenshots and graphics to PDF.

• Image to PDF

  Mix JPG, PNG, HEIC, WebP and more in one batch.