Highlights and regions

Highlighting passages in Theca is more than just marking text: what you mark is saved to your account and can become the source for an AI-generated summary. It is how you build an active reading record from your books.

Highlighting a passage

1. Open the book in the reader.
2. Select the text you want to mark with your cursor (in PDF) or with touch selection (in EPUB or TXT).
3. Choose a color from the picker that appears.

The passage is highlighted and saved automatically.

Available highlight colors

The picker offers five colors:

• Green
• Blue
• Pink
• Purple
• Red

You can change the active color at any time from the Highlight button in the toolbar. When a color is active, the button shows that color; click it again to deactivate highlighting.

Erasing a highlight

The color picker also has an eraser button (Eraser mode). With eraser mode active, dragging over an existing highlight removes it. If the erased area covers only part of a highlight, the rest of the highlight is kept.

Viewing all highlights in a book

Highlights are stored per book. You can see them directly in the reader: every marked passage stays visible with its color as you navigate through the pages.

Recognized formulas and chemistry (Regions)

In PDF books that contain mathematical or chemical notation, Theca can detect and extract those regions automatically. If the book has them, the Formulas & chemistry button appears in the reader toolbar. Clicking it opens a side panel listing each region, its type (Formula or Chemistry), the page it appears on, and an image of the original snippet alongside the rendered notation.

Some regions may carry a Review label, which means it is worth checking them against the original source.

The highlights you make in a book are what Theca's summary system can use as quotable passages. The more precise what you mark, the better the summary result.

Frequently asked questions

Are highlights saved when I close the reader? Yes. Theca saves each highlight immediately to your account.

Can I highlight in EPUB and TXT as well as PDF? Yes. Highlighting works in all three supported formats: PDF, EPUB, and TXT.

How are highlights grouped? Highlights are linked to the book where you created them. They are not grouped by color or chapter in a separate view; they remain visible on the corresponding page when you read the book.

What exactly are formula and chemistry "regions"? They are areas of the PDF where Theca detected mathematical or chemical notation. They are extracted automatically when the book is processed and rendered clearly so you can read them, alongside the original snippet from the page.

Next step

• Navigating the reader — page controls and table of contents in the reader.
• Summaries — use your annotated books as a source to generate an AI summary with traceable citations.
• Study cards to EON — turn passages from your summaries into review flashcards.