How to Merge Multiple Images into a Single PDF

Merging a folder of scanned receipts, photos, or screenshots into a single PDF is a common task with a surprisingly tricky edge: images come in different sizes, orientations, and aspect ratios. The simple Pillow approach uses each image as a PDF page at its native resolution — fine for uniform screenshots but awkward for mixed-size images. This guide covers both the quick approach and the professional approach with consistent A4 pages.

TL;DR

• Quickest: Pillow save_all=True — each image becomes one page at native resolution
• Consistent page size: reportlab or fpdf2 — scale images to fit A4/Letter with margins
• Mixed formats (HEIC + JPG + PNG): load with Pillow, convert all to RGB, then merge
• API: POST individual images to convert to PDF, then merge with pypdf

Pillow: quick merge (native resolution)

pip install Pillow

from PIL import Image, ImageOps
from pathlib import Path

def merge_images_to_pdf(image_paths: list[str], out_pdf: str) -> int:
    """
    Merge a list of image files into a single PDF.
    Each image becomes one page at its native resolution.
    """
    pages = []
    for p in image_paths:
        img = Image.open(p)
        img = ImageOps.exif_transpose(img)  # fix phone photo rotation
        if img.mode in ("RGBA", "P", "LA"):
            bg = Image.new("RGB", img.size, (255, 255, 255))
            if img.mode == "RGBA":
                bg.paste(img, mask=img.split()[3])
            else:
                bg.paste(img.convert("RGBA"), mask=img.convert("RGBA").split()[3])
            img = bg
        elif img.mode != "RGB":
            img = img.convert("RGB")
        pages.append(img)

    if not pages:
        raise ValueError("No images to merge")

    pages[0].save(out_pdf, save_all=True, append_images=pages[1:])
    return len(pages)

# Merge all images in a folder (sorted alphabetically)
folder = Path("./receipts")
image_files = sorted(
    [p for p in folder.iterdir() if p.suffix.lower() in (".jpg", ".jpeg", ".png", ".bmp", ".tiff")]
)
count = merge_images_to_pdf([str(p) for p in image_files], "receipts.pdf")
print(f"Merged {count} images into receipts.pdf")

A4-consistent pages with reportlab

For professional documents where all pages should be A4 (or Letter) with consistent margins:

pip install reportlab Pillow

from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import A4, LETTER
from PIL import Image, ImageOps
from pathlib import Path
import io

A4_W, A4_H = A4  # 595.27 x 841.89 points (1pt = 1/72 inch)
MARGIN = 40  # points

def merge_to_a4_pdf(image_paths: list[str], out_pdf: str, pagesize=A4) -> int:
    w, h = pagesize
    content_w = w - 2 * MARGIN
    content_h = h - 2 * MARGIN

    c = canvas.Canvas(out_pdf, pagesize=pagesize)
    count = 0

    for path in image_paths:
        img = Image.open(path)
        img = ImageOps.exif_transpose(img)
        if img.mode != "RGB":
            bg = Image.new("RGB", img.size, (255, 255, 255))
            if img.mode == "RGBA":
                bg.paste(img, mask=img.split()[3])
            else:
                bg.paste(img.convert("RGB"))
            img = bg

        # Scale to fit within content area, maintain aspect ratio
        iw, ih = img.size
        ratio = min(content_w / iw, content_h / ih)
        draw_w = iw * ratio
        draw_h = ih * ratio

        # Center on page
        x = MARGIN + (content_w - draw_w) / 2
        y = MARGIN + (content_h - draw_h) / 2

        # Write image to temp bytes
        buf = io.BytesIO()
        img.save(buf, "JPEG", quality=88)
        buf.seek(0)

        c.drawImage(buf, x, y, draw_w, draw_h)
        c.showPage()
        count += 1

    c.save()
    return count

images = sorted(Path("./scans").glob("*.jpg"))
merge_to_a4_pdf([str(p) for p in images], "invoice-package.pdf")

ChangeThisFile API + pypdf merge

# Convert individual images to PDF
curl -X POST https://changethisfile.com/v1/convert \
  -H "Authorization: Bearer ctf_sk_your_key_here" \
  -F "file=@image1.jpg" -F "target=pdf" --output p1.pdf

curl -X POST https://changethisfile.com/v1/convert \
  -H "Authorization: Bearer ctf_sk_your_key_here" \
  -F "file=@image2.jpg" -F "target=pdf" --output p2.pdf

# Then merge locally with pypdf
pip install pypdf

import requests
from pypdf import PdfWriter
from pathlib import Path
import tempfile

API_KEY = "ctf_sk_your_key_here"

def images_to_pdf_via_api(image_paths: list[str], out_pdf: str) -> int:
    writer = PdfWriter()
    with tempfile.TemporaryDirectory() as tmp:
        for i, img_path in enumerate(image_paths):
            with open(img_path, "rb") as f:
                resp = requests.post(
                    "https://changethisfile.com/v1/convert",
                    headers={"Authorization": f"Bearer {API_KEY}"},
                    files={"file": f},
                    data={"target": "pdf"},
                    timeout=30,
                )
            resp.raise_for_status()
            tmp_pdf = Path(tmp) / f"page_{i:03d}.pdf"
            tmp_pdf.write_bytes(resp.content)
            writer.append(str(tmp_pdf))

    with open(out_pdf, "wb") as f:
        writer.write(f)
    return len(image_paths)

Edge cases and gotchas

• Page ordering. Sort by filename or pass an explicit ordered list. alphabetical sort works for zero-padded numbers (001.jpg, 002.jpg). File modification time is unreliable for ordering after copying.
• HEIC images. Pillow doesn't read HEIC natively — use pyheif first (see bulk-convert-heic-to-jpg guide) or convert via API before merging.
• Very large images (4K+). A 4000×3000 image in a PDF inflates it significantly. Resize to max 2048px on the longest side before merging unless you specifically need archival quality.
• Multi-page TIFFs. Some TIFF files contain multiple pages (common for fax output). ImageSequence.Iterator handles multi-page TIFFs — each TIFF page becomes a PDF page.
• Image DPI metadata and PDF page size. Pillow uses image DPI metadata to set PDF page size. A 300 DPI 3000×2100px image becomes a 10×7 inch PDF page. For consistent A4 output, use the reportlab approach which explicitly sets the page size.

Processing scan batches

# Merge scans from multiple subfolders, one PDF per folder
from pathlib import Path

base = Path("./scan-archive")
for folder in sorted(base.iterdir()):
    if not folder.is_dir(): continue
    images = sorted([p for p in folder.iterdir() if p.suffix.lower() in (".jpg", ".png", ".tiff")])
    if not images:
        print(f"Skipping {folder.name}: no images")
        continue
    out = folder.parent / f"{folder.name}.pdf"
    count = merge_images_to_pdf([str(p) for p in images], str(out))
    size_mb = out.stat().st_size / 1e6
    print(f"{folder.name}: {count} images → {out.name} ({size_mb:.1f}MB)")

For receipts and casual docs, the Pillow approach is 5 lines and done. For anything going to a client or legal file, use the reportlab A4 approach to get consistent, professional page layout. API free tier handles HEIC and mixed-format inputs without local conversion deps.