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Reduce PDF Size on Windows, Mac, and Mobile

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Reduce PDF Size: Free Tools and Step-by-Step GuideReducing PDF size can save storage, speed up uploads, and make sharing by email or web easier. This guide covers why PDF files get large, free tools you can use across platforms, and step‑by‑step instructions for several common methods so you can pick the one that fits your workflow.

Why PDFs become large

Large PDFs are usually caused by:

  • High-resolution images embedded without compression.
  • Embedded fonts and multiple font subsets.
  • Unoptimized scanned pages (each page stored as a full image).
  • Redundant or excessive metadata, annotations, and form data.

Understanding the cause helps choose the right method: compress images, subset or remove fonts, convert scans to optimized text via OCR, or strip unnecessary data.

Quick checklist before you start

  • Keep an original backup copy.
  • Decide acceptable quality loss (lossy vs. lossless).
  • Choose whether you need to preserve vector quality (for diagrams) or can rasterize pages.
  • If file is a scanned document, plan to run OCR after compression if you need searchable text.

Free tools you can use (cross-platform)

  • Smallpdf (web) — simple, browser-based compression with options for stronger compression (lossy) or basic compression (less quality loss).
  • ILovePDF (web) — similar to Smallpdf; intuitive and fast for batch jobs.
  • PDF24 Tools (web & desktop for Windows) — good balance of features; desktop version works offline.
  • Sejda (web & desktop) — allows per-file choices and limited free daily usage.
  • Ghostscript (command-line, Windows/Mac/Linux) — powerful, scriptable, fully free; ideal for advanced users.
  • qpdf (command-line) — best for linearization and basic optimization, not image re‑compression.
  • LibreOffice (desktop) — open-source office suite that can export to PDF with adjustable image compression and quality.
  • Preview (Mac) — built-in macOS app with simple export/compress options and Quartz filters.
  • PDFsam Basic (desktop) — for splitting, merging, and rearranging pages, which can indirectly reduce size.
  • ImageMagick (command-line) — convert pages to images at chosen DPI; useful when rasterizing is acceptable.

Method 1 — Web tools (fast, easy, cross-platform)

Best when you don’t want to install anything and file isn’t sensitive.

Steps (generic):

  1. Open the web tool (Smallpdf, ILovePDF, Sejda, or PDF24).
  2. Upload your PDF file.
  3. Choose compression level (recommended: start with “basic/medium” to balance quality and size).
  4. Download the compressed PDF.
  5. Verify visual quality and test searchability if the file contains text.

Pros: no install, quick.
Cons: upload required (avoid for sensitive documents) and free limits on file size/number.

Method 2 — PDF optimization in desktop apps

Use when you need offline processing or more control.

A. LibreOffice (export with compression)

  1. Open your document in LibreOffice (or import PDF using Draw for direct edits).
  2. File → Export As → Export as PDF.
  3. In the PDF Options dialog, under “Images” set JPEG compression and lower quality (e.g., 75%). Reduce resolution (e.g., 150 dpi) if acceptable.
  4. Click Export and compare file sizes.

B. macOS Preview

  1. Open the PDF in Preview.
  2. File → Export.
  3. Choose “Reduce File Size” Quartz Filter.
  4. Save under a new name and check results.
    Tip: Quartz filters can be customized in ColorSync Utility to adjust compression settings.

C. PDF24 Creator (Windows desktop)

  1. Install PDF24 Creator.
  2. Open the PDF in PDF24 or drag it to the tool.
  3. Choose “Compress” and pick a preset or custom compression level.
  4. Save and inspect.

Method 3 — Command-line (power and automation)

Best for batch jobs, servers, or advanced customization.

A. Ghostscript (recommended command) Windows/Mac/Linux — install Ghostscript, then run:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4     -dPDFSETTINGS=/ebook     -dNOPAUSE -dQUIET -dBATCH     -sOutputFile=output.pdf input.pdf

Common PDFSETTINGS:

  • /screen — low-resolution, smallest size (72 dpi images)
  • /ebook — medium (150 dpi)
  • /printer — higher quality (300 dpi)
  • /prepress — high quality, larger size
  • /default — balanced default settings

Adjust by replacing /ebook with desired setting. For finer control, add:

  • -dDownsampleColorImages=true -dColorImageResolution=150

B. qpdf (linearize or rebuild structure)

qpdf --stream-data=compress --recompress-flate input.pdf output.pdf

qpdf is not an image compressor but can reduce overhead and optimize object streams.

Method 4 — Replacing or recompressing images manually

If images are the main cause, extract, recompress, and reinsert.

  1. Extract images with tools like pdfimages (part of poppler-utils) or using some PDF editors.
  2. Recompress images in batch with ImageMagick or jpegoptim (e.g., reduce quality to 75% and resize).
  3. Replace images using a PDF editor (or rebuild PDF from images using ImageMagick/LibreOffice).

Example ImageMagick command to resize/recompress:

magick input.jpg -resize 1400x -quality 75 output.jpg

Rebuild PDF:

magick *.jpg -quality 85 output.pdf

Method 5 — OCR scanned PDFs and save as text + images

For scanned documents where each page is a large image, run OCR and produce a PDF with a text layer and compressed images.

  1. Use tools like OCRmyPDF (command-line; includes Ghostscript internally).
  2. Command example: 
    
ocrmypdf --optimize 3 input_scanned.pdf output_ocr.pdf

    Optimization levels adjust image compression and resolution.

Tips to preserve usability and quality

  • For documents with line art or vector diagrams, avoid aggressive JPEG compression—prefer higher DPI or lossless compression for those pages.
  • If fonts bloat the file, export with font subsetting or remove embedded fonts when acceptable.
  • Use linearization (“fast web view”) if the PDF will be served online; it doesn’t always reduce size much but improves page‑by‑page loading.
  • Test different settings on a copy, not the original. Compare visual quality and file size.
  • Emails and uploads: /ebook (Ghostscript) or medium compression in web tools (≈150 dpi).
  • Printing: /printer (300 dpi) or minimal compression.
  • Archival with full fidelity: avoid lossy compression; consider ZIP or keep original.

Troubleshooting

  • If size doesn’t change: file may already be optimized or contains many vectors/fonts; try Ghostscript’s different PDFSETTINGS or re-create PDF from source.
  • If text becomes unreadable: revert to a higher dpi/less aggressive compression.
  • If OCR fails: try improving scan contrast or use higher-resolution scans before OCR.

Example workflow (scanned report to email-ready PDF)

  1. Run OCRmyPDF with optimization: 
    
ocrmypdf --deskew --clean --optimize 3 scanned.pdf scanned_ocr.pdf
  2. If still large, run Ghostscript medium compression: 
    
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook  -dNOPAUSE -dQUIET -dBATCH -sOutputFile=final.pdf scanned_ocr.pdf
  3. Verify text is searchable and visual quality acceptable.

Reducing PDF size is often a trade‑off between quality and capacity. Use the tool and settings that match your needs — web tools for quick jobs, desktop apps for privacy, and command‑line tools for automation and fine control.

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