A paper is a long, figure-heavy PDF, and PDF is one of the four formats whose rendering has been verified on a clean device in FoldRender. This is the procedure for getting a paper onto an unfolded Fold and reading it properly — from a browser download, an email attachment or the repository the paper's code lives in — plus a straight account of what a reader-only app does not give you as a researcher.
FoldRender is on the Play Store as FoldRender: Repo Reader, package id dev.foldrender.app, free and distributed worldwide. Install it the ordinary way — no allow installs from this source prompt to approve, and updates arrive on their own. The app declares minSdk 30, so it needs Android 11 or newer; Play will tell you if a device is below that rather than letting you install a build that cannot run.
However the PDF arrives — saved from the browser, an email attachment, a file in Drive or Downloads — it lands in ordinary Android storage. Tap it in the Files app and choose Render in the “Open with” list: FoldRender registers for application/pdf by MIME type and for .pdf by extension, so it is offered even by providers that serve the file with a vague content type. From inside the app, the folder icon in the top bar — or Open a file on the empty screen — picks the same file through the system document picker.
If the paper has a public repository — reference implementations live on GitHub all over research — clone it: branch glyph in the top bar, paste the HTTPS URL into Clone an HTTPS repository URL, tap Clone. A public repository clones anonymously, with no account and no token. A PDF checked into that repository opens from the same tree as the source, which is the whole point: the figure on page 7 and the function that produced it, one screen apart.
In the Files tab every row carries a type badge, and PDFs are badged PDF in red — easy to spot in a repository of source files. Unfolded, the tree stays on the left and the paper renders on the right; folded, the paper takes the whole screen and the tree becomes a back step.
The header above the page strip reads N pages · page M, and M updates live as you scroll. A horizontal strip of page thumbnails sits alongside the document; tapping a thumbnail jumps the main list to that page — the working substitute for skimming back to figure 3, then forward again.
The text-size icon opens the Reading sheet: Theme (System / Light / Dark), a Text size slider from 80% to 160%, a Line width slider from 480 dp to 1200 dp, and a serif switch. Pages re-render at the new width rather than upscaling a stale bitmap — rotating or refolding does not blur the page. One-page-at-a-time rendering behind a mutex keeps memory flat even on a 200-page document.
A PDF opened from local storage needs no connection, and every renderer in the app works on the local file. A paper read on a plane behaves exactly like a paper read at a desk.
PDF goes through Android's own PdfRenderer — no native libraries added to the APK, and pages look the way the rest of the system renders them. Detection scores name, MIME type and magic bytes together, so a paper whose filename lost its extension still opens as a PDF if its first bytes are %PDF.
The same pane renders the Markdown README, the highlighted source, the notebooks and the PDF, from one tree. For a paper with a public implementation, that beats a standalone PDF viewer: the equation and the line of code that implements it are a thumbnail tap apart.
Reading a local PDF involves no network, and even inside a cloned repository the only operations that touch the remote are git ones — Pull, for when the authors push a revision.
The honest list — shipped-but-unverified behaviour included, so you can tell before installing whether this build does the thing you came for.