feat(notes): add guide for OCR and PDF manipulation on Linux

- Added instructions for setting up Tesseract with language support.
- Documented steps for converting PDFs to images using `pdftoppm` and alternatives like `ImageMagick`.
- Included examples for single and multi-page OCR processing.
- Detailed methods for merging extracted text into a single file.
- Added troubleshooting tips for improving OCR results and handling selectable PDFs with `pdftotext`.

notes/pdf.md

@@ -0,0 +1,104 @@
+# PDF
+
+## Table of Contents
+
+- [PDF](#pdf)
+  - [Table of Contents](#table-of-contents)
+  - [OCR](#ocr)
+
+## OCR
+
+**Install necessary tools**
+Update the package list and install Tesseract with support for the desired language. Replace `fra` with your desired language code (e.g., `eng` for English).
+
+```bash
+apt update
+apt install tesseract-ocr tesseract-ocr-fra
+```
+
+**Verify the installation**
+
+Check the installed version of Tesseract and list available languages to ensure your chosen language is installed (e.g., `fra` for French).
+
+```bash
+tesseract --version
+tesseract --list-langs
+```
+
+**Install a utility to convert PDF pages into images**
+
+For PDFs that require OCR, you need a utility to convert PDF pages into images. Install `poppler-utils`, which includes `pdftoppm`.
+
+```bash
+apt install poppler-utils
+```
+
+**Convert PDF to images**
+
+Convert the PDF into JPEG images, with each page saved as a separate file. Each page will be named sequentially (e.g., `output-1.jpg`, `output-2.jpg`, etc.).
+
+```bash
+pdftoppm -jpeg your_file.pdf output
+```
+
+- **Tip**: Use a dedicated output directory to avoid overwriting existing files.
+- **Alternative tools**: If `poppler-utils` isn’t available, consider using `ImageMagick`:
+
+  ```bash
+  convert -density 300 your_file.pdf output-%04d.jpg
+  ```
+
+**Perform OCR on a single image**
+
+Run Tesseract OCR on an image to extract text. Specify the language using the `-l` option.
+
+```bash
+tesseract output-1.jpg output-text -l fra
+```
+
+The extracted text will be saved in `output-text.txt`.
+
+**Perform OCR on multiple images**
+
+For multi-page PDFs, process all images in a loop. This extracts text from each image and saves it to a separate `.txt` file.
+
+```bash
+for img in output-*.jpg; do
+    tesseract "$img" "${img%.jpg}" -l fra
+done
+```
+
+- **Note**: The `${img%.jpg}` syntax removes the `.jpg` extension, ensuring each `.txt` file matches its corresponding image.
+
+**Combine all text files**
+
+Merge the text from all processed pages into a single file. This is useful for assembling the full content of the PDF.
+
+```bash
+cat output-*.txt > complete_text.txt
+```
+
+If filenames are out of order, use a sorting approach before merging:
+
+```bash
+ls output-*.txt | sort -V | xargs cat > complete_text.txt
+```
+
+**Troubleshooting and Tips**
+
+**If Tesseract doesn’t recognize text**:
+
+- Ensure the images have sufficient quality and resolution. Use `-r` with `pdftoppm` to increase the DPI (e.g., `-r 300` for 300 DPI).
+- Try additional Tesseract language packs for better recognition of specific text styles.
+
+**Using `pdftotext` for simpler PDFs**:  
+ If the PDF contains selectable text (not just images), `pdftotext` from `poppler-utils` can extract text directly without OCR.
+
+```bash
+pdftotext your_file.pdf output.txt
+```
+
+Replace `convert` with `magick` for newer versions.
+
+**Verify language codes for Tesseract**:  
+ You can find a list of supported languages on the [Tesseract GitHub page](https://github.com/tesseract-ocr/tesseract).