Upload a workbook to the Excel viewer. Every sheet opens, tab by tab.

XLS, XLSX, CSV, ODS, XLSM, XLSB, XLTX and XLTM. 20 MB per file. Deleted after viewing. Excel to PDF, Excel to JPG and Excel to PNG take XLS and XLSX, up to 5 MB.

What each extension is

XLSX — current format, a ZIP of XML. XLS is the pre-2007 binary one.
XLSB — the same modern workbook saved as binary. Faster, much smaller.
XLSM / XLTM — macros attached. They render, they do not run.
XLTX — a template.
ODS — LibreOffice and OpenOffice.
CSV — one sheet of text. No formulas, no formatting, no tabs.

A CSV that arrives all in column A

The file used a delimiter your locale does not expect: semicolons where commas were assumed, or tabs. Open it in Excel through Data, From Text/CSV and set the delimiter by hand, or save it again as XLSX and upload that.

Rows and sheets you cannot see

Three things hide data without breaking anything.

AutoFilter. Rows are hidden, not removed. The giveaway is the row numbers jumping.
Hidden sheets. Right-click any tab, Unhide, in Excel.
Very hidden sheets. Set through VBA and absent from the Unhide list entirely.

Exporting a wide sheet to PDF

A sheet has no natural page. Set the print area and the page breaks in Excel before converting, or a 40-column report becomes a PDF sliced into strips. Page Layout, Print Area, then Print Titles to repeat the header row on every page.

When the upload fails

Over 20 MB. Usually a whole-column conditional format or an oversized used range. Ctrl+End shows you where Excel thinks the data stops; delete the rows below it and save.

Password required to open. Type the open password into the field under the upload box and the workbook renders. Without it, nothing can.

Protected sheet or protected workbook structure. A flag, not encryption. Opens normally.

Cells full of #####. The column is narrower than the number. Widen it in the source file.

Sending the numbers to somebody who has no spreadsheet software? Excel to PDF is the short way.