Add Watermark to PDF – Frequently Asked Questions

The Watermark PDF tool is a free online service that adds watermarks (text or images) to your PDF documents for protection and branding.

Key Features:

• Nine Positions: 3x3 grid (top/middle/bottom × left/center/right)
• Opacity Control: 0-100% transparency
• Scale Adjustment: 10-200% sizing
• Rotation: -180° to +180° angle control
• Batch Processing: Watermark multiple PDFs at once
• ZIP Download: Download all watermarked PDFs in one ZIP file
• Base64 Images: Secure image handling

Technical Implementation: Upload a watermark image (logo, text image, etc.) which is converted to base64 format. The server uses FPDI to import PDF pages and TCPDF's Image() method to embed the watermark at the specified position with opacity, scale, and rotation settings. For batch watermarking, the same settings are applied to all uploaded PDFs, and results are packaged into a ZIP archive.

The tool offers nine position options in a 3×3 grid covering the entire page.

Position Grid:

• Top Row:
  • Top-Left (upper left corner)
  • Top-Center (centered at top)
  • Top-Right (upper right corner)
• Middle Row:
  • Middle-Left (centered vertically on left)
  • Center (exact center of page) – most common for prominent watermarks
  • Middle-Right (centered vertically on right)
• Bottom Row:
  • Bottom-Left (lower left corner)
  • Bottom-Center (centered at bottom)
  • Bottom-Right (lower right corner)

Use Cases by Position:

• Center: Prominent watermarks (DRAFT, CONFIDENTIAL)
• Bottom-Right: Company logos, copyright notices
• Top-Right: Document identifiers, version numbers
• Corners: Discrete branding

Opacity controls the transparency of your watermark from completely invisible to fully opaque.

Opacity Range: 0% to 100%

• 0%: Completely invisible (no watermark visible)
• 10-30%: Very subtle, background-style watermark
  • Use Case: Non-distracting protection, maintains document readability
• 40-60%: Visible but translucent watermark
  • Use Case: Standard watermarking, balanced visibility
• 70-90%: Strong, clear watermark
  • Use Case: Emphasize document status (DRAFT, COPY)
• 100%: Fully opaque (solid watermark)
  • Use Case: Maximum visibility, prominent branding

Technical Details:

• Opacity is applied using alpha transparency in the image embedding
• TCPDF applies opacity when rendering the watermark image
• Lower opacity allows document content to show through clearly

Scale controls the size of your watermark from 10% to 200% of its original dimensions.

Scale Range: 10% to 200%

• 10-30%: Very small watermark
  • Use Case: Tiny logos, discrete branding
• 40-60%: Small to medium watermark
  • Use Case: Corner logos, small text watermarks
• 70-100%: Standard size watermark
  • Use Case: Normal watermarking, balanced size
• 110-150%: Large watermark
  • Use Case: Prominent watermarks, full-width text
• 160-200%: Very large watermark
  • Use Case: Diagonal full-page watermarks (DRAFT, CONFIDENTIAL)

Scaling Behavior:

• Proportional Scaling: Maintains aspect ratio (no distortion)
• 100% = Original Size: Watermark at its uploaded dimensions
• Relative to Page: Final size depends on page dimensions and watermark image size

Yes! Rotation ranges from -180° to +180° for complete angle control.

Rotation Angles:

• 0°: Horizontal (normal orientation)
  • Use Case: Standard logos, horizontal text
• 45°: Diagonal from bottom-left to top-right
  • Use Case: Classic "DRAFT" or "CONFIDENTIAL" diagonal watermark
• -45°: Diagonal from top-left to bottom-right
  • Use Case: Alternative diagonal orientation
• 90°: Vertical (rotated right)
  • Use Case: Vertical text along page edge
• -90° / 270°: Vertical (rotated left)
  • Use Case: Opposite vertical orientation
• 180°: Upside down
  • Use Case: Special layouts, creative designs

Rotation Features:

• Center Point: Rotation occurs around watermark center
• Any Angle: Set precise custom angles (e.g., 33°, -67°)
• Combination: Combine rotation with position, opacity, and scale

Yes! The tool supports batch watermarking of multiple PDFs simultaneously.

How Batch Watermarking Works:

1. Upload Watermark Image: Select your watermark (logo, text image, etc.)
2. Configure Settings: Set position, opacity, scale, rotation
3. Upload Multiple PDFs: Select all PDFs you want to watermark
4. Process: Same watermark settings applied to all PDFs
5. Download: All watermarked PDFs packaged in a ZIP file

Batch Features:

• Same Settings: All PDFs get identical watermark
• Individual Processing: Each PDF processed separately with same parameters
• ZIP Download: Convenient single-file download for all results
• Time Saver: Much faster than watermarking PDFs one by one

Use Cases:

• Watermark entire document sets with company logo
• Add "CONFIDENTIAL" to multiple files
• Batch brand documents for distribution

When watermarking multiple PDFs, all results are packaged into a single ZIP archive for convenient download.

ZIP Download Process:

1. Batch Processing: All PDFs are watermarked individually
2. ZIP Creation: Server creates a ZIP archive containing all watermarked PDFs
3. Naming: ZIP file named like watermarked_pdfs_[timestamp].zip
4. Download: Single download link for the ZIP file
5. Extract: Unzip on your computer to access all watermarked PDFs

ZIP Archive Contents:

• Each watermarked PDF named: watermarked_[original_filename].pdf
• All files in one ZIP (no folders, flat structure)
• Original filenames preserved with "watermarked_" prefix

Benefits:

• Single Download: One click instead of downloading 10+ PDFs individually
• Organization: All watermarked files in one place
• Convenience: Extract to any folder on your computer

Base64 encoding converts your watermark image to a text string for secure transmission and processing.

How Base64 Encoding Works:

1. Upload: You upload a watermark image (PNG, JPG, etc.)
2. Encoding: Browser converts image to base64 data URL
   • Format: data:image/png;base64,iVBORw0KGgoAAAANS...
3. Transmission: Base64 string sent to server (not the binary image file)
4. Decoding: Server decodes base64 back to image data
5. Embedding: Image data embedded in PDF

Why Use Base64?

• No Temporary Files: No need to save image files on server
• Security: Image data processed in memory, not on disk
• Simplicity: Easy to transmit via web forms
• Compatibility: Works in all browsers and with all image types

Technical Details:

• Base64 increases data size by ~33% (acceptable trade-off for convenience)
• Supports all image formats (PNG, JPG, GIF, etc.)
• Decoding happens server-side before PDF embedding

Yes! The Watermark tool is fully responsive and mobile-friendly.

Mobile Features:

• Image Upload: Upload watermark images from photo library or camera
• PDF Upload: Select PDFs from device storage
• Setting Controls: Touch-friendly sliders and dropdowns for position, opacity, scale, rotation
• Batch Processing: Watermark multiple PDFs on mobile
• ZIP Download: Download ZIP archives on mobile devices

Mobile Tips:

• Works on iOS (iPhone, iPad) and Android devices
• Processing happens server-side (no performance impact)
• For large batches, desktop may be more convenient for managing downloads

The Watermark tool uses advanced PDF and image processing technologies.

Backend Technologies:

• FPDI: Imports PDF pages from source files
• TCPDF Image() Method: Embeds watermark images on pages
  • Position calculation based on 9-position grid
  • Opacity/alpha transparency settings
  • Scale and rotation transformations
• Base64 Encoding/Decoding:
  • Frontend: Image → base64 data URL
  • Backend: base64 → image data
• ZIP Archive Creation: For batch downloads
  • PHP ZipArchive class
• Batch Processing: Loop through multiple PDFs with same settings

Position Calculations:

• Nine Positions (3×3 grid):
  • X: left = margin, center = (width/2), right = width - margin
  • Y: top = margin, middle = (height/2), bottom = height - margin
• Opacity: 0-100% alpha transparency
• Scale: 10-200% proportional sizing
• Rotation: -180° to +180° transformation matrix

Processing Workflow:

1. User uploads watermark image → converted to base64
2. User sets position, opacity, scale, rotation
3. User uploads PDF(s)
4. Server processes each PDF:
   • Import pages with FPDI
   • Decode base64 watermark image
   • Calculate position coordinates
   • Apply opacity, scale, rotation
   • Embed watermark using TCPDF Image()
5. For batch: package all watermarked PDFs into ZIP
6. Return download link(s)