Augmented Reality (AR) promises to enhance daily office activities involving numerous textual documents, slides, and spreadsheets by expanding workspaces and enabling more direct interaction. However, there is a lack of systematic understanding of how knowledge workers can manage multiple documents and organize, explore, and compare them in AR environments. Therefore, we conducted a user-centered design study (N = 21) using predefined spatial document layouts in AR to elicit interaction techniques, resulting in 790 observation notes. Thematic analysis identified various interaction methods for aggregating, distributing, transforming, inspecting, and navigating document collections. Based on these findings, we propose a design space and distill design implications for AR document arrangement systems, such as enabling body-anchored storage, facilitating layout spreading and compressing, and designing interactions for layout transformation. To demonstrate their usage, we developed a rapid prototyping system and exemplify three envisioned scenarios. With this, we aim to inspire the design of future immersive offices.