I would recommend any device on which KOReader runs. These currently include Android, Cervantes, Kindle, Kobo, Pocketbook, ReMarkable and desktop Linux. (Some of them require first a jailbreak before KOReader can be installed.)

KOReader is a viewer program for all the common e-book formats, but I will here describe only its PDF viewer, because this is most important for mathematicians. And the PDF viewer is much more powerful that all the built-in PDF viewers that I know. It has the following features (among others):

• Easy cropping, in a semi-automatic or an automatic mode. One can remove page margins and display only the text of a PDF file.
• The cropped pages can be arranged in a continuous flow.
• KOReader can handle multicolumn texts reasonably well. It then only shows a smaller region of a page, containing part on a single column, and when one requests the next “page”, the displayed region moves in the right way.
• The display of the text can be rotated by 90°, which helps when a paper has rather long lines.
• Text can also be reflowed, and mathematical formulas are preserved. (Reflow does not always work well, but that is a general problem with algorithms that need to “guess”.)
• Many PDFs one can get from the internet do not contain metadata for a table of content. But most of them contain at least an OCR layer. In such cases, one can mark the chapter and section headings on the screen: KOReader generates bookmarks for them which contain the text of the headings, and the bookmarks menu serves as an acceptable table of contents. (This is an incredibly helpful feature!)

KOReader is under a very active development: Currently there are monthly updates.