Moving your writing in and out of Nodes is meant to be easy in both directions. You can bring an existing collection with you, and you can take everything with you whenever you like.
Import
Already have a pile of markdown files or PDFs? Choose File ▸ Import… (⇧⌘I) and the Import dialog opens with a drop zone: Drop files or a Nodes archive. What you give it decides what happens - loose Markdown, PDF, or image files become full notes in your library, ready to link, tag, and search, while a .nodes archive restores a library: nodes, tags, and links. Below the drop zone, browse files opens a picker that takes several files at once, and open archive picks a .nodes archive directly.
An archive isn't imported the moment it lands. The dialog stages it for review first, showing a manifest of what's inside - how many nodes, tags, and stickers - and nothing happens until you commit with the Import N Nodes button. If some of those notes already live in your library, the review step counts them and offers three choices: Keep Both (the default - your version stays untouched and the incoming one arrives as a copy), Replace, or Skip. The dialog closes on Esc or a click outside it - unless an import is mid-write, in which case it stays put until it finishes.
Got just one loose file to deal with? Open it in a Light Window and choose Import into Nodes.
Drag and drop
Dragging is the fastest way to bring in a single file. Drag a PDF, Markdown, or image file from Finder onto the workspace window and a Drop to Import overlay appears. Release, and the file becomes a full note in your library - ready to link, tag, and search, just like any other. It opens where you dropped it: in the tab you dragged onto, or in a new tab when you drop onto Home.
Drop something Nodes can't read and it tells you the type isn't supported. Stick to PDF, Markdown, or image files and you're set. One special case: drop a .nodes archive and Nodes opens the Import dialog with the archive staged for review instead of importing it on the spot - an archive is a whole library, not a single note.
Export
To export notes as files, choose File ▸ Export… (⇧⌘E). With a note open, the dialog is already set up to export that note; export more nodes… leads to the full picker, where you can select any set of notes. Markdown notes leave as Original .md or as a Typeset PDF - with Letter or A4 paper and a font-size control, and a live page preview so you see the result before saving. PDF and image notes simply export as their original files. A single note saves through a save panel; several notes are written as plain files into a folder you choose, each named after its note's title. It's the real content in open formats, not a locked proprietary database.
To back up the whole library, go to Settings ▸ General ▸ Library Backup and click Save Archive…. That writes everything into a single .nodes archive - every note, tag, link, description, and sticker - so a later import restores your library exactly.
Keep that archive as a portable backup, or move it wherever you like. It's the half of Library Backup that leaves this Mac; the other half, library snapshots, keeps daily recovery points here without you having to remember anything.
Why it's built this way
Your notes are plain files to begin with (see Where your files live), so export is really just handing you a tidy copy of what's already yours. There's no lock-in by design: you should stay with Nodes because you want to, never because you can't leave.