How to Convert Images to PDF for Research — Figures, Gels and Microscopy
Journal submissions almost always require figures as high-resolution PDFs or TIFFs rather than JPEGs or PNGs. Whether you are submitting gel images, microscopy photographs, chromatograms or graphs, getting the format right matters. A rejected figure file can delay your submission by days. This guide explains what journals want and how to convert your images correctly — for free.
What Journals Actually Require
Most journals specify figure requirements in their author guidelines. The most common requirements across major publishers (Elsevier, Springer, Wiley, Nature, PLOS) are:
| Figure Type | Minimum Resolution | Preferred Format |
|---|---|---|
| Line art (graphs, diagrams) | 1000 dpi | EPS, PDF, or TIFF |
| Halftone (photos, gels, microscopy) | 300 dpi | TIFF or PDF |
| Combination (graph + photo) | 500 dpi | TIFF or PDF |
| Colour mode | RGB for online, CMYK for print | Check journal guidelines |
⚠️ Common Mistake
Saving a figure as JPEG at any stage — even temporarily — introduces compression artefacts that cannot be removed. Always work with lossless formats (PNG, TIFF) throughout your workflow and only convert to JPEG if the journal explicitly requires it.
Understanding Resolution — DPI vs Pixel Dimensions
DPI (dots per inch) only matters in combination with the physical print size. A 300 dpi image that is 3.5 inches wide has 1050 pixels across. If you enlarge it to 7 inches wide, the effective resolution drops to 150 dpi — below the threshold.
Most journal figures are printed at 86 mm (single column) or 178 mm (double column) width. At 300 dpi, that means a minimum of 1016 × 2102 pixels respectively. Check your image dimensions before submission — not just the file size.
How to Convert PNG or JPEG Images to PDF — Free Methods
Method 1 — Scitero Image to PDF (Browser-based, Free)
The fastest option for converting single or multiple images to PDF directly in your browser. No software installation, no upload to external servers, works on any device.
- Upload your PNG, JPEG, or TIFF files
- Arrange in the correct order
- Download as a single PDF instantly
Method 2 — ImageJ / FIJI (Free, Recommended for Scientific Images)
ImageJ is the gold standard for scientific image processing and is free. It handles TIFF, LSM, CZI (Zeiss), LIF (Leica) and most microscopy formats natively.
- Open your image in ImageJ (File → Open)
- Set scale and check image type (Image → Type → should be RGB Color for colour images)
- Set resolution: Image → Scale → enter target DPI
- Export: File → Save As → TIFF (lossless) or use Print to PDF
Method 3 — Adobe Acrobat / Preview (Mac)
On Mac, simply open your PNG or TIFF in Preview, then File → Export as PDF. This preserves resolution without re-compressing the image. On Windows, the Microsoft Print to PDF option works similarly but does not preserve DPI metadata — set your printer DPI to 600 before printing.
Gel Images — Special Considerations
Gel photographs have specific requirements because they are evidence of experimental results. Journals have strict policies on acceptable image processing.
- Crop only to remove irrelevant lanes — and disclose in the figure legend that the image was cropped
- Do not adjust brightness or contrast selectively — any adjustment must be applied equally to the whole image
- Never use JPEG for gel images — JPEG compression creates artefacts around bands that can be misinterpreted as additional bands
- Include a molecular weight marker lane in every gel figure
- State the staining method in the figure legend (Coomassie, silver, ethidium bromide, etc.)
⚠️ Watch out
Image manipulation is one of the most common reasons papers are retracted after publication. Nature and other journals use image forensics software to detect splicing, cloning and contrast manipulation. When in doubt, keep the original raw image file and document every processing step.
Microscopy Images
Confocal, fluorescence, and brightfield microscopy images have their own workflow considerations.
- Export from your microscope software (Zeiss ZEN, Leica LAS, Nikon NIS) as TIFF, not the proprietary format
- For fluorescence overlays — create RGB composite images with ImageJ using Image → Color → Merge Channels
- Add scale bars using ImageJ: Analyze → Set Scale, then Analyze → Tools → Scale Bar
- Always state in the legend: magnification, scale bar length, and fluorescent channel colours
- For z-stacks, include the slice position and step size in the methods section
File Size — Compressing Without Losing Quality
High-resolution figures can be very large. Many journals have a total submission size limit of 10–50 MB. If your figures are too large:
- Save TIFF files with LZW compression (lossless — reduces size 30–50% without quality loss)
- For PDF figures, compress using Scitero PDF Compressor — it reduces file size while preserving print quality
- Convert to PDF and compress before uploading, not after review
Convert Images to PDF — Free
Convert PNG, JPEG and TIFF images to PDF for journal submission. Works in your browser, no upload to external servers.
Open Image to PDF Tool →