Add readme

Author: itsbjoern

README.md

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+# PDF Interpolator
+
+A desktop application for replacing text in PDF files using data from spreadsheets.
+
+## Overview
+
+PDF Interpolator reads a spreadsheet (Excel or CSV) and replaces matching text in a PDF document. You define which columns to use: one column contains the text to find, another contains the replacement text.
+
+The application preserves the original PDF formatting, fonts, and layout while performing the replacements.
+
+## Download
+
+Download the latest version for your operating system from the [releases page](https://github.com/itsbjoern/pdf-interpolator/releases).
+
+**Available for:**
+- Windows (.exe installer)
+- macOS (DMG)
+- Linux (AppImage, snap, deb)
+
+## How to Use
+
+### 1. Select a Spreadsheet
+
+Click "Select Spreadsheet" and choose your Excel or CSV file. If your spreadsheet has multiple sheets, select which ones you want to use.
+
+### 2. Set Up Column Mappings
+
+For each sheet, specify two columns:
+- **Source Column**: Contains the text to search for in the PDF
+- **Target Column**: Contains the replacement text
+
+Example: If your spreadsheet has "Old Name" and "New Name" columns, the app will find each "Old Name" value in the PDF and replace it with the corresponding "New Name" value.
+
+### 3. Select the PDF
+
+Choose the PDF file you want to process.
+
+### 4. Choose Output Location
+
+Select where to save the modified PDF.
+
+### 5. Process
+
+Click "Process PDF" to start. The application will show a detailed results window with:
+- Total matches found
+- Successful replacements
+- Any encoding issues (if characters in your replacement text are not available in the PDF's fonts)
+
+## Requirements
+
+- The spreadsheet must contain at least two columns per sheet (one for finding text, one for replacing)
+- Text matches are case-sensitive
+- Empty cells are skipped
+
+## Known Limitations
+
+- Special characters in replacement text must be available in the PDF's fonts
+- Very large PDFs may take longer to process
+- Text spanning multiple lines or text segments may not be replaced
+
+## Building from Source
+
+```bash
+# Install dependencies
+npm install
+
+# Run in development mode
+npm run dev
+
+# Build for production
+npm run build
+
+# Build platform-specific installers
+npm run build:win    # Windows
+npm run build:mac    # macOS
+npm run build:linux  # Linux
+```