Ebook and 3D File Conversion Guide

Two niche but high-search-intent conversion areas: ebook formats (PDF ↔ EPUB ↔ MOBI for reading on e-readers) and 3D printing formats (STL ↔ OBJ ↔ 3MF for slicing). Both have specific gotchas free converters often gloss over. This guide covers what to use, when, and what to expect.

Ebook format overview

The major ebook formats and what each is for:

• EPUB: open-standard, reflowable. Adjusts to screen size — text wraps, font scales. Used by Apple Books, Kobo, Adobe Digital Editions, most ebook readers except Kindle.
• MOBI: Amazon’s older Kindle format. Mostly deprecated in 2026 — Amazon stopped accepting MOBI uploads in late 2022. Convert old MOBI to EPUB (or AZW3) for forward compatibility.
• AZW3 / KF8: Amazon’s current Kindle format. Better typography than MOBI. Newer Kindles support it.
• KFX: Amazon’s newest format. Used by recent Kindles. Partially supported by Calibre + plugins; full support is difficult because the format is proprietary.
• PDF: not really an ebook format. Fixed-layout means it looks bad on small screens. Avoid for e-readers if you have any other choice.

The universal converter: Calibre (free, open-source). Drag in your file, pick output format, get the result. Calibre handles almost every ebook format combination including PDF→EPUB.

Converting PDF to EPUB for e-readers

The workflow:

1. Install Calibre. Free, available on Mac, Windows, Linux.
2. Drag the PDF into Calibre. It appears in your library.
3. Click Convert Books. Select EPUB as the output format.
4. Adjust settings. The defaults are good. For PDFs with complex layout, try the “PDF Input” tab and toggle “No images” if you want text-only.
5. Click OK. EPUB lands in your library. Send to your e-reader.

Caveats:

• Multi-column PDFs convert poorly. Calibre tries to flatten columns into single-column reflowable text but the order can scramble.
• Image-heavy PDFs become large EPUBs. Compress images first if size is a concern.
• Forms and interactive elements don’t convert.
• For Kindle, convert PDF → EPUB → AZW3 (Calibre handles all three), then send via Send-to-Kindle email or USB.

Converting scientific PDFs with math symbols

Math notation in PDFs is usually rendered as embedded fonts or images. The conversion challenge:

• Calibre PDF → EPUB: preserves math as embedded images (the rendered glyphs). Readable, but you can’t edit or re-render.
• Mathpix Snip: commercial, but converts math images into editable LaTeX. Best for academics needing to extract equations.
• arXiv source: if the paper is on arXiv, download the LaTeX source instead of the PDF. Compile to EPUB with Pandoc:pandoc paper.tex -o paper.epub --mathml. Math renders natively in EPUB readers that support MathML.
• Pandoc + custom workflow: for high-volume scientific document conversion, Pandoc can take Markdown with LaTeX math and produce EPUB / DOCX / PDF preserving the math.

The honest answer: scientific PDF → readable e-reader format works OK for casual reading. For editing or re-rendering math, you need the LaTeX source (or pay Mathpix).

3D printing format overview

• STL (Stereolithography): the universal 3D printing format. Just a triangle mesh — no color, no texture, no metadata. Every slicer accepts it. Default for hobbyist 3D printing.
• OBJ: mesh + texture coordinates + vertex colors. Used for 3D rendering and game development; sometimes for color 3D printing.
• 3MF (3D Manufacturing Format): Microsoft + 3D printing consortium format. Includes mesh, color, materials, multi-part assemblies. The modern replacement for STL — but adoption is slower than expected.
• STEP / IGES: CAD interchange formats. Parametric models, not just meshes. Used in engineering. Most consumer slicers can’t read STEP — convert to STL first.
• G-code: the slicer’s output, the printer’s input. Not a CAD format — printer-specific machine instructions.

Converting STL to OBJ for 3D printing

Most 3D modeling tools handle this round-trip. The free options:

• Blender: File → Import STL → File → Export OBJ. Preserves geometry, lets you add textures and vertex colors before export.
• MeshLab: minimal viewer-converter for mesh formats. Drag-and-drop conversion. Smaller install than Blender.
• FreeCAD: import STL, export to OBJ. Useful if you also need to edit the geometry.
• Online converters: upload STL, download OBJ. Convenient but uploads your model. Avoid for proprietary or commercial designs.

The conversion itself is straightforward — both formats represent a triangle mesh, OBJ adds texture coordinates which STL doesn’t have. What can go wrong:

• Scale mismatch. STL doesn’t encode units. OBJ does. The converter has to guess. Verify the size in the slicer.
• Triangle order. Some tools reverse winding order which can flip normals. Check with normals visualization in your slicer.
• Vertex precision. STL stores vertices as binary floats. OBJ uses ASCII text. Round-trip can introduce tiny precision errors — usually invisible but rarely problematic for ultra-precise prints.

Converting 3MF without losing 3D printing details

3MF carries more information than STL: per-part materials, colors, multi-part assemblies, embedded thumbnails. Converting 3MF → STL throws this away. The strategies:

• If your slicer accepts 3MF: just use 3MF directly. Cura, PrusaSlicer, Bambu Studio all support it. No conversion needed.
• If you must convert to STL: export each part as a separate STL file. Loses the multi-part assembly metadata but preserves geometry. Re-assemble in the slicer.
• 3MF to OBJ: Blender + the 3MF import addon, then export OBJ. Preserves colors and materials better than STL.
• For sharing: 3MF is the right modern format. Keep your master in 3MF; export to STL only when targeting old slicers.