Somewhere between converting a PDF and opening the result, you get asked to pick a file format. XLS or XLSX. It looks like a setting nobody should have to think about, and then a workbook silently drops forty thousand rows on save, or an old accounting system refuses the file you just spent an hour cleaning up. The two formats are genuinely different under the hood, and the difference occasionally costs you data.
The short answer
XLS is the original Excel binary format, used by default from Excel 97 through Excel 2003, and it holds a maximum of 65,536 rows and 256 columns per sheet. XLSX has been the default since Excel 2007 and is a zipped collection of XML files rather than one binary blob, holding up to 1,048,576 rows and 16,384 columns per sheet in a noticeably smaller file. Use XLSX unless a specific program or template will only accept .xls.
What is the difference between XLS and XLSX?
The practical differences come down to five things: how the file is stored, how much it can hold, how big it is on disk, whether it can carry macros, and what still opens it.
| XLS | XLSX | |
|---|---|---|
| Default in | Excel 97 to 2003 | Excel 2007 and later |
| Structure | Single binary file (BIFF) | Zipped folder of XML files (Office Open XML) |
| Max rows per sheet | 65,536 | 1,048,576 |
| Max columns per sheet | 256 (through IV) | 16,384 (through XFD) |
| Typical file size | Larger | Smaller, often by half |
| Can contain macros | Yes | No, macros need .xlsm |
| Opened by modern Excel | Yes | Yes |
| Opened by Google Sheets | Yes, with quirks | Yes |
Microsoft's own specifications page gives the modern worksheet limit as 1,048,576 rows by 16,384 columns. That figure is the same across Excel for Microsoft 365, Excel 2024, 2021, 2019, and 2016. The 65,536 number attached to XLS is not a soft guideline either. It is the hard edge of the format.
Is XLSX better than XLS?
For almost everyone, yes. XLSX holds sixteen times the rows and sixty four times the columns, produces smaller files, and is the format every current spreadsheet program expects. It is also easier to recover when a file corrupts, because it is really a zip archive of readable XML rather than one opaque binary block. The only reason to choose XLS is compatibility with something old that will not take anything else.
That compatibility case is real, though, and it is not rare in finance and operations. Line of business systems, older accounting packages, some payroll and banking upload tools, and internal templates built fifteen years ago still list .xls as the only accepted import format. If that is your situation, XLS is not a mistake, it is a requirement.
The 65,536 row limit is the one that actually bites
This is the failure worth remembering. If you have a sheet with more than 65,536 rows and you save it as .xls, everything past that line is gone. Excel warns you, but the warning arrives in a dialog at the end of a long working session, and it is easy to click through while thinking about something else. The file saves, it opens fine, and the missing rows only surface weeks later when a total does not match.
The same trap applies to columns at 256, though far fewer people build sheets that wide. It also applies to features: conditional formatting rules, table styles, and newer functions can degrade or vanish when a workbook is saved down to the older format, because the format has nowhere to store them.
If you are regularly near that ceiling, the data is usually arriving from more documents than one file can comfortably hold. At that volume it is worth pulling the documents into structured data in bulk instead of converting and stitching them one at a time. Within a single large source file, our guide to converting a large PDF to Excel covers how long documents are handled without splitting the table.
Why is my XLSX file smaller than my XLS file?
Because XLSX is compressed. The format is a zip archive containing XML documents, and XML text compresses extremely well. Save the same workbook both ways and the XLSX version is frequently around half the size, sometimes better on sheets with a lot of repeated formatting. Nothing is lost in that reduction; it is ordinary compression, not a simplification of your data.
You can see this for yourself. Copy an .xlsx file, rename the copy to end in .zip, and open it. Inside are the worksheets, styles, and shared strings as separate XML parts. That transparency is also why XLSX plays nicely with programming languages and reporting tools, which can read and write it without Excel installed.
What about XLSM and XLSB?
Two more extensions turn up in the same conversation, and they exist for specific reasons.
| Extension | What it is | Use it when |
|---|---|---|
| .xlsx | Standard modern workbook, no macros allowed | The default for essentially everything |
| .xlsm | Same format, macros permitted | The workbook contains VBA you need to keep |
| .xlsb | Modern limits stored in binary rather than XML | Very large workbooks that open and save slowly |
| .xls | Legacy binary, 65,536 row cap | Only when a system demands it |
| .csv | Plain text, one sheet, no formatting | Moving raw data between programs |
The split between .xlsx and .xlsm is a security decision by Microsoft rather than a technical one. Both are the same underlying format. Separating them means a file ending in .xlsx cannot carry macro code, so opening one is safe by inspection, and anything that might run code announces itself in the extension.
XLSB is worth knowing about if you work with genuinely heavy models. It stores the same modern row and column limits in a binary form, which can open and recalculate faster than the XML equivalent. The tradeoff is that fewer non Excel tools read it cleanly.
XLS vs XLSX vs CSV: which should I use?
CSV is a different kind of thing. XLS and XLSX are both workbook formats that keep sheets, formatting, formulas, and data types. CSV is a plain text list of values with one sheet, no formatting, and no formula support. It is the right choice when a file is going into another program rather than in front of a person, because it carries nothing for the receiving system to misread.
The catch is that CSV has no concept of data types, so it quietly damages certain values: leading zeros disappear from account and pro numbers, long identifiers get rendered in scientific notation, and dates reformat according to whatever locale opens them. Our breakdown of CSV versus Excel covers exactly what gets lost and how to prevent it. If a system is asking you for CSV specifically, convert straight to CSV rather than exporting an XLSX and saving down.
How do I convert XLS to XLSX?
Open the .xls file in any current version of Excel and choose File, then Save As, then pick Excel Workbook (.xlsx) as the type. That is the whole process, and it is lossless in that direction because you are moving to the format with the larger limits. Excel may also offer a Convert button in the File, Info panel when it detects a workbook opened in compatibility mode, which does the same job in one click.
Going the other direction, XLSX to XLS, is the one to be careful with. Save As, then Excel 97 to 2003 Workbook (.xls). Excel runs a compatibility check first and tells you what will not survive. Read that dialog rather than clicking past it, particularly the row count line.
Can Excel still open XLS files?
Yes. Current versions of Excel open .xls files without any add in, though they open in compatibility mode, which restricts you to the older format's limits and features while that file is active. Some organizations block older Office formats through Trust Center policy for security reasons, which is why an .xls attachment occasionally refuses to open on a managed work laptop. That is a policy setting rather than a broken file, and IT can confirm it.
Which format should a PDF converter give you?
XLSX, in almost every case. When you convert a PDF into a spreadsheet you want the result to carry real data types, numbers that are numbers and dates that are dates, across as many rows as the document needs. XLSX does that and imposes no practical ceiling on a converted document.
Our converter outputs a modern Excel file that opens in any current version of Excel or Google Sheets. If you specifically need the legacy binary, convert first and then use Save As, Excel 97 to 2003 Workbook, which takes one click and carries your columns and numbers across intact. The two format pages walk through each path: converting PDF to XLSX for the modern format, and converting PDF to XLS when an older system is the constraint.
Whichever format you land on, the check that matters is the same one: after converting, confirm a numeric column actually sums. A column that looks like numbers but returns zero from SUM is text, and our guide to converting text to numbers in Excel fixes it in a pass. Format choice decides where your data can live. Data types decide whether it can be used.